Are all the Computers / Monitors close to the UPS?
Or do you intend connect 3 Computers, 3 Monitors, 1 printer, 1 Broadband router by running extension cable to each computer from the UPS.
NOT RECOMMAND FIRE RISK
Please note that to run three Computer System, Modem/Router/Printer/ plus any other electrical equipment would required a very large UPS system
It would be best if you had a UPS for each Computer / Monitor System
With three UPS System each would shut down gracefully on it own