3 sticks of 512ram problem

I have been running 2 Samsung PC3200 512meg sticks of ram in slots 1 and 3 in my Abit NF7-S rev2 in Dual channel mode for 3 months with no problem at 210x11.5 for 2415mhz.
I have a spare stick of Samsung PC2700 512meg that I have tried to put in for a total of 1.5 gig with no success. I have even tried stock settings like 133x14 with no luck. It sees the ram and just starts to boot and locks up.
Is it not possible to mix ram like this? Can anyone shed any light on this.
Thanks.

By putting in the PC2700 ram you are making your PC3200 sticks run at 2700 speeds. Also how many ram slots do you have? If it's 3 then you can only run a max of 2 sticks at a time on most mobos.

I realize running a stick of PC2700 with the two PC3200 sticks will make me only have PC2700 speeds, but I thought I could run 3 sticks of ram total if I had 3 slots.
This motherboard has 3 slots available.