1734-TOP is the one-piece screw-terminal base with 8 terminals.
1734-TOP3 is the one-piece screw-terminal base with 12 terminals.
It is my understanding that the number "3" means that the module was intended for use with "3-wire sensors", because it has enough +24V and DC Common terminals for four sensors to be connected with +24, DC Common, and Input conductors.
In general, you can use a 1734-TOP3 for an application that would also be suitable for a 1734-TOP.