Testing the water in the 3g should give you as much information as you need here.
I think if there's any 'sickness' to be concerned with, it would be what he has already and may introduce to the 37g tank, the biggest issue (I assume) would be ammonia levels in the 3g tank, frequent water changes should mitigate that, testing the water would let you know if there is or is not a need to change the water.
you don't say how big this fish is, nor the dimensions of the tank he's in. so it may be fine, you don't really give enough information to know that, so advice will be given based on worst case scenarios to try to make this as easy on the fish as possible.
you don't need to be providing a tank that's big enough for a full grown adult fish at this point, just one that's big enough for the size of the fish at the time he will be removed. If the filter wasn't seeded, you are basically going through a fish-in cycle at the moment
I think if there's any 'sickness' to be concerned with, it would be what he has already and may introduce to the 37g tank, the biggest issue (I assume) would be ammonia levels in the 3g tank, frequent water changes should mitigate that, testing the water would let you know if there is or is not a need to change the water.
you don't say how big this fish is, nor the dimensions of the tank he's in. so it may be fine, you don't really give enough information to know that, so advice will be given based on worst case scenarios to try to make this as easy on the fish as possible.
you don't need to be providing a tank that's big enough for a full grown adult fish at this point, just one that's big enough for the size of the fish at the time he will be removed. If the filter wasn't seeded, you are basically going through a fish-in cycle at the moment