You can get away with one in a 5g, but getting a 10 gallon and adding two or three would be better. Keep in mind that these crabs are brackish water animals. That means they live in an environment that has roughly half the salinity of ocean water. To make brackish water, you need to use good quality marine salt, not table salt. The prefer a specific gravity of about 1.008.
Also, like you mentioned, they do need space to climb out. The need to be able to fully come out of the water, and preferably have areas of land to explore. There are several ways of doing this. You can make a raised platform that sticks out of the water, and have branches or something to access the platform. You can also make a half sand/half water setup, where you and a lot of sand and slope it to create a shore. Then you half fill the tank with water.
I've kept these guys in the past, so let me know if you need more help. You can take a look at the crab website in my signature. It's about fiddler crabs, but much of the requirements are the same.
--Adeeb