MacBook – Dual (DVI) monitor setup for MacBook Pro 15″ (Retina mid-2015)

You've pretty much got two options for two external monitors on a MacBook Pro:

A) Two Apple Thunderbolt Displays

This is the least flexible, but highest performing solution. Obvious drawbacks are the cost, and not being able to use your existing displays. This only works on Thunderbolt equipped Macs with a discrete graphics card (i.e. not the 13" MacBook Pro or any MacBook Air, nor the base Mini).

B) DisplayLink adapter

You can use one display directly connected to the Thunderbolt/Mini Displayport with an mDP-HDMI adapter, and run the second one with a DisplayLink adapter. DisplayLink is essentially basic graphics card connected to your computer via USB. Because of the limitations of USB, it's not a high performance solution - don't expect to do any gaming on it. This does allow you to use your existing monitors, and at relatively modest expense. You may also want to check the current status of Lion support, as I remember seeing some posts about bugs relating to DisplayLink adapters - they may have been fixed by now however.

You could run 2 DVI/VGA montiors via the thunderbolt ports with adapters, and 1 via HDMI. I believe the 4th would need to be connected with a USB-to-vdeo adapter.

Check out this question for some more information: How many external displays can I power with the 15" retina MacBook Pro?