C3 is gonna be very yellow. If N18 is your true match, C3 won't be. But aside from the colour, i definitely don't suggest laying the two. Studio fix isn't a powder, it's a full coverage powder foundation, meant to be worn alone, not in addition to another foundation. So you either wear the pro longwear foundation, or the studio fix, not both. The PLW doesn't need set, as it sets itself and won't move, unlike other MAC foundations. If you're looking for a setting powder, try either the P&P translucent, MSFN, or PRO set powder.