Due to dag file size issue, 3x 750ti will be able to mine ETH, efficiently ?
If you are running hard memory algo such as lyra/neoscrypt/ (ethminer, but I think we can forgot it with this card), you will require that much memory (or alternatively pagefile).
This is how cuda works:
The allocation to the GPU memory proceed through the pre-allocation of the space address memory (?? I love using jargon, I don't understand entirely ) on the RAM. So say you want to allocate 2Gb of GPU mem, you'll need 2GB of ram or equivalent (pagefile system) for the pre-allocation.
Hence you DO need the same amount of RAM. (I have been saying it like forever... )