The tank could definitely support 3 hermit crabs, probably more. My 13.5 has 8. So long as there are enough shells that they don’t fight each other, they should be alright. They don’t produce too much waste, so bioload ain’t a major issue.
Be careful they will quickly kill your snails all types of hermits if you don't have bigger open shells if you don't have any next time at your LFS look in the tanks ask to buy some bigger ones that are empty then clean them and drop them in I have 6 hermits in my 32gal they have all different types of shells now!
When crabs are added, snail population decreases.
It’s a trade off.
I’ve never used a crab in 10 years now, there not the greatest cleaners IMM.
But I use a high diversity in snails to compensate, and I still have them all.