Since 2008 I use two monitors in my desktop. Yesterday I bought a new graphics interface and a third monitor. Some time I was looking for a low cost graphics interface. Ok, I am using GeForce GT 740 which has three output ports: VGA, DVI and HDMI. In Brazil this interface card can be found around R$ 400 (US$ 117, but my card was US$ 87 in Brazilian Black Friday). In Amazon.com, it is between US$ 51 and US$ 109. The chosen manufacturer was Zotac, but all GT 740 and 750 will work fine (I tested the GT 750 too).
The GeForce GT 740 was imediatelly recognised by Debian Jessie with kernel Linux 4.7.0 from Backports (it is my default, so I didn't test with original 3.16 kernel). The driver used was the default X.Org Nouveau. I use KDE and the management was easy.
I hope this post can help people interested in use 3 monitors. Enjoy!