Most of the time 3 x Titans will have the lead but in some games, 2 x 980 Ti can match or beat 3 x SLI Titans. Hoever, one must also take into account that the 980 Ti can overclock as much as 31% over Nvidia reference.....the Titan X isn't as overclockable at 4.1%
Again, in order to recommend a monitor a resolution is required
But, no at the settings generally used by most gamers, 3 x SLI does not get you 150 fps at any resolution
http://www.maximumpc.com/nvidia-geforce-gtx-titan-x-sli-benchmarks-2015/
Titan X 3-way SLI @ 4k
Metro LL = 53 fps
Arkham Origins = 110
Hitman Absolution = 78
Shadow of Mordor = 89 (980 Ti in 2 way SLI gets 89.5)
Tomb Raider = 119 w/ TressX
The situation you are at now is that any monitor you can buy at $400 would be very poorly matched to that system.
With the three Titans, I would be looking at the following:
Acer Predator Z35 (200 Hz, 2560 x 1080) - ?
http://www.tftcentral.co.uk/reviews/acer_predator_z35.htm
Asus ROG Swift PG279Q - $800
http://www.tftcentral.co.uk/reviews/asus_rog_swift_pg279q.htm
http://pcpartpicker.com/part/asus-monitor-pg279q
Acer Predator XB270HU - $720
http://www.tftcentral.co.uk/reviews/acer_xb270hu.htm
http://pcpartpicker.com/part/acer-monitor-xb270hubprz
Asus ROG Swift PG278Q - $670
http://www.tftcentral.co.uk/reviews/asus_rog_swift_pg278q.htm
http://pcpartpicker.com/part/asus-monitor-90lm00u0b01370