900mhz 3 90*, 4 90* or 3 120* Antennas?

Will I be shooting myself in the foot trying to use three 90’s on a tower (Til-tek TA-926VH-4-90 ) instead of 4 90’s, 3 120’s etc.

Also, how much better can I expect NLOS penetration to be using sectors as opposed to a 11dBi Omni?

I don’t know how well you’ll cover the gaps, but I can tell you that deploying four of those where we had an omni has been wonderful. We found out that we have noise that is pretty directional, that is now impacting about 12 clients instead of over 100. We have full 360 degree coverage and then some, as we have a few of the clients that are closer connecting “around the corner” so to speak to get them off of the rough sector, so you may be all right. By the way, if your concern is mounting to a three-legged tower, that’s what we did with four radios. I can take a snapshot if you’d like. The worst part is that one of the antennas is tilted about 4 degrees.

With 3 90’s you will have nulls where the coverages don’t overlap.

I’d recommend the MTI 120 H-POL - these antennas are amazing