When I have all three cards out, if there are no creatures on the board with P/T higher than two, it's a board wipe. However, it's also infinite creature creation with the Nest of Scarabs popping out 2 tokens with each kill. Or is it equalized because state-based actions are accounted for between the two triggers of blowfly infestation? What if you have 2 Nest of Scarabs out? What about Hapatra, vizier of poisons? What if they have two toughness, say from bad moon?
When you put one or more -1/-1 counters on a creature while there are no other creatures, Nest of Scarabs's ability will trigger. If it would be destroyed or go to the graveyard for having lethal damage and/or toughness 1 or less, that creature will do so as a state-based action (C.R. 704.5f-g, 116.3b, 116.5), making each Blowfly Infestation's ability trigger (C.R. 603.2). However, as you put the Blowfly Infestation abilities on the stack, if you can't target any creatures, they will be removed from the stack (C.R. 603.3d, 601.2c), leaving only the Nest of Scarabs ability. And you will still create the number of tokens stated by that ability when that ability resolves. If you control more than one Nest of Scarabs, each will trigger and get to resolve accordingly. The same applies if you controlled Hapatra rather than Nest of Scarabs.
When you put one or more -1/-1 counters on a creature while there is at least one other creature, the same happens, except you must target a creature on the battlefield if possible for each Blowfly Infestation ability you put on the stack this way (C.R. 603.3d, 601.2c). (Note that since you control the Blowfly Infestation abilities on the stack, you can order them there along with any other triggered abilities you control that would go on the stack at the same time as them [C.R. 603.3b, 112.8], and one or more Blowfly Infestation abilities can target the same creature.) When each Blowfly Infestation ability resolves, if the target is still legal (C.R. 608.2b), you must put a -1/-1 counter on that target, and the process continues according to this and the preceding paragraph.
What is giving the creatures the initial -1/-1 counter?
Let's break down what (I think) you are trying to do:
You control a Nest of Scarabs and 2 Blowfly Infestations. Your opponent controls 3 creatures: a 1/1, a 2/2, and a 3/3. You cast Black Sun's Zenith with X=2. Now, 2 counters go onto each of their creatures causing you to create a total of 6 tokens. Also, 2 of their creatures die with -1/-1 counters so both Infestations trigger. However, the only creature left on the board (assuming you don't have any) is the 3/3 with 2 -1/-1 counters on it. You must target that creature with both triggers. Then, it will die and, if you stacked the triggers so you got the creatures from Nest before getting to put counters on a creature from Blowfly Infestations, you will have 6 creatures to target with the 2 new triggers from Blowfly Infestation. This will cause more tokens from Nest and then cause more triggers from the Blowflys and so on. It seems that you will end up getting "infinite" tokens.
Note that this does rely on there being one creature alive after the initial "board wipe". In the above scenario, if the opponent had 3 1/1 creatures, they would all die and there would be no creatures alive to continue putting counters onto thus stopping the loop. You can't put counters on the creatures that would be created by Nest with the trigger from the Blowflys if they trigger at the same time.
Hapatra works basically the same way, though she can target herself as part of the loop to keep it going.
When you put one or more -1/-1 counters on a creature while there are no other creatures, Nest of Scarabs's ability will trigger. If it would be destroyed or go to the graveyard for having lethal damage and/or toughness 1 or less, that creature will do so as a state-based action (C.R. 704.5f-g, 116.3b, 116.5), making each Blowfly Infestation's ability trigger (C.R. 603.2). However, as you put the Blowfly Infestation abilities on the stack, if you can't target any creatures, they will be removed from the stack (C.R. 603.3d, 601.2c), leaving only the Nest of Scarabs ability. And you will still create the number of tokens stated by that ability when that ability resolves. If you control more than one Nest of Scarabs, each will trigger and get to resolve accordingly. The same applies if you controlled Hapatra rather than Nest of Scarabs.
When you put one or more -1/-1 counters on a creature while there is at least one other creature, the same happens, except you must target a creature on the battlefield if possible for each Blowfly Infestation ability you put on the stack this way (C.R. 603.3d, 601.2c). (Note that since you control the Blowfly Infestation abilities on the stack, you can order them there along with any other triggered abilities you control that would go on the stack at the same time as them [C.R. 603.3b, 112.8], and one or more Blowfly Infestation abilities can target the same creature.) When each Blowfly Infestation ability resolves, if the target is still legal (C.R. 608.2b), you must put a -1/-1 counter on that target, and the process continues according to this and the preceding paragraph.
Let's break down what (I think) you are trying to do:
You control a Nest of Scarabs and 2 Blowfly Infestations. Your opponent controls 3 creatures: a 1/1, a 2/2, and a 3/3. You cast Black Sun's Zenith with X=2. Now, 2 counters go onto each of their creatures causing you to create a total of 6 tokens. Also, 2 of their creatures die with -1/-1 counters so both Infestations trigger. However, the only creature left on the board (assuming you don't have any) is the 3/3 with 2 -1/-1 counters on it. You must target that creature with both triggers. Then, it will die and, if you stacked the triggers so you got the creatures from Nest before getting to put counters on a creature from Blowfly Infestations, you will have 6 creatures to target with the 2 new triggers from Blowfly Infestation. This will cause more tokens from Nest and then cause more triggers from the Blowflys and so on. It seems that you will end up getting "infinite" tokens.
Note that this does rely on there being one creature alive after the initial "board wipe". In the above scenario, if the opponent had 3 1/1 creatures, they would all die and there would be no creatures alive to continue putting counters onto thus stopping the loop. You can't put counters on the creatures that would be created by Nest with the trigger from the Blowflys if they trigger at the same time.
Hapatra works basically the same way, though she can target herself as part of the loop to keep it going.