They are by a good margin the slowest growing fish I've ever encountered.
They also get on quite well as a group, and I'd say you'll be fine with them for a time to come.
Eventually they'll reach a stage (about 3") when a pair will likely form out of them. At this point it is best to remove the extra's and give the pair the tank. These guys are not that active or large and you'll be able to keep a pair for years in a 3x1 tank, and can include some carefully selected tankmates too.
FYI There is a dwarf version known as a Sombu Dwarf that only gets to 2-3" long would work great for life in a tank this size with other smaller tankmates like shelldwellers.