You can use the older SP3 dock which has a single mini displayport, and the SP3 has another mini displayport. You would need a minidisplayport to DVI or VGA, DVI would be a better choice of course, you can find them on amazon or monoprice for maybe 20 bucks or so.
You can also use the newer SP4 dock with your SP3, the dock itself has 2 mini displayports. The main difference is the SP3 port lets you stand the SP3 up in it, the SP4 port is more of a brick. The SP4 port also allows a bit higher resolution if you are running 4k monitors, otherwise you won't notice the difference. If you're not running 4k monitors I'd probably try to get a SP3 dock for cheap now that they are probably on fire sale.