So do you now have 2
pairs of RAM chips? Or one pair?
Running three chips (even if all the same speed) will switch the MoBo into a slower RAM access mode. Fast RAM chips should always be fitted in pairs.
For example, let's say that you already had 2x DDR3 1333 MHz chips totalling 2 GB (i.e. 2x 1 GB chips), but wanted to expand this for a 32 bit OS. You could fit another pair of 2x 1 GB DDR3 1333 MHz chips, giving you a total of 4 GB RAM.
If you already had 2x 2GB chips and were running a 64 bit OS, you could do either the above, fit a matching pair of 2 GB chips (total of 6 GB RAM), or fit another 2x 4GB chips (same specs) to give you a total of 10 GB. Both should work perfectly well, as the RAM speed, type and size allows the OS to mirror its access to each pair of chips. All this is dependent on what the MoBo manual states is allowed, but this sort of configuration is generally allowable in all the MoBos I have ever used.
If you fitted one of each type into the wrong pair of slots, the MoBo might either reject all the RAM outright, or run as slow as a dog with three broken legs ... It would be highly unlikely that this configuration would work at all, or work properly, if it did work. In this case, the correct way to install the chips would be to have the two 4 GB chips in slots 1 & 3 and the two 1 or 2 GB chips in slots 2 & 4 - this assumes that slots 1 & 3 are one colour, and slots 2 & 4 are the other colour.
I hope this makes some kind of sense - it's been a long day ...
.