It very much depends on what model of 3750 switch you are using; there is a large variety of different models.

Particularly relevant is the fact that some versions support Power over Ethernet (for powering IP Phones and Wireless Access Points).

To err on the safe side, I'm assuming you're using a 48-port PoE model of the 3750, and that all 3 switches are identical.

At maximum load, the WS-C3750-48PS-S will draw 540W from the UPS - the current will be between 4 and 8 Amps.

My UPS Power Calculator works out that you would need at least a 4.4kVA UPS to keep these devices running in the event of loss of mains power.

You should speak to your UPS vendor to see what options you have for such a setup, as UPSs vary considerably also - you don't want to choose a 4.4kVA UPS and then find out that it will only power your switches for a couple of minutes before the batteries run out!

Hope this helps

Graeme