Can I tie into a three-phase #10 circuit with #12 wire?
Feeding common NEMA 5-15 receptacles (plural), the circuit (i.e. the breaker) must be 15A or 20A.  You may not put these receptacles on a 30A circuit (breaker)!   

You're always allowed to use larger wire.  A 20A circuit calls for 12AWG wire minimum, but you may use 10AWG. 

So most likely you must change the breaker serving the wires to 120V/20A, then extend with 12AWG wire. 

Safety first

Be very wary of strange and dangerous voltages including 136V, 208V, 277V or 480V.  These voltages will be happy to kill you.  If you don't know what you're doing in commercial panels, get out of them and call an electrician.  

Add more wire

Since you are commercial, and therefore in conduit, you always have the option to fish additional wires into the conduit. You can have up to 4 circuits per conduit without complications.  Pro tip: You have a choice of like 11 wire colors, so use colors different than those already there.