Once you forward a port you can not forward it again.
So, you might do this...
#1 Computer one port 2222
b) Computer two port 2223
c) Computer three port 2224
#2 In the router forward port 2222 to computer one
b) In the router forward port 2223 to computer two
c) In the router forward port 2224 to computer three
#3 As need be...
a) You forward port 22 to computer one.
b) Once you know it works/the port is open from the net, then you re-edit the rule and forward port 22 to computer two.
c) Sort of Again. Once you know it works/the port is open from the net, then you re-edit the rule and forward port 22 to computer three.