Shimano 9 speed shifters
You should use Shimano mountain bike shifters (Acera, Alivio, Deore, etc.) which have the same # of chainrings and same # of cogs on the cassette. So if you're running a triple in the front and a 9 speed cassette in the back, you want a 3x9 shifter set. The road bike ones (Sora, Tiagra, 105, Claris, Ultegra, etc. - even flat bar ones, like the Sora flat bars) will not work because they use different cable pull in the front. 

You can't use SRAM shifters other than the SRAM Attack line (which are friction front, indexed rear, so you'll see the ones for a 3 x 9 setup just marketed as a 9 speed setup), since they have different cable pulls. 

Microshift also make ones compatible with Shimano as well in the XE Marvo line (but I don't know anyone who has actually used a Microshift shifter on a mountain bike - usually its just their bar ends on a Surly road bike or similar).