Based on what you have written here I would suggest you skip them. First, snakes live for 20+ years easily, you don't really have any help and it sounds like they are pets that you just want to watch so why do you need any more then the 3 you have. Also, the fact one "seemed to have disappeared" like it is no big deal gives me red flags that more snakes is probably not a good idea...I would suggest you just enjoy the ones you have and give them the best care possible before you quickly outgrow the novality and get more than you really want to care for and feed weekly.
Just my 2 cents.