As I stated (and Xpert as well), Boxer Glue is the way to go - not wally world glue - there's a reason we don't buy wally world stuff.
A 3" gash would need a 5x3" patch with rounded corders, if you have an overlap liner, you can trim some of the excess and use it to patch - sometimes you can even match the pattern. Sacrificing a bag (used for inground pools) is a relatively cheap alternative, asking at the pool store if they have any excess liner material will sometimes yield free results - many times, pool stores will warranty liners, and only have to send the rip in for warranty purposes, leaving them with a rather large piece of useless vinyl.
With regards to "can you patch it dry"? Of course you can, however, if it's on the floor I don't recommend it since the pool should NEVER be emptied, if it's on a wall - go for it.