Can I use a DC 60W 18v 3A solar pannel to power my macbook pro 15" retina mid-2012?
Ok ,the math goes like this

Volt x Amps = Watts

First important factor is the Voltage. It has to be the right one for the circuits otherwise they burn up. You would not connect 120 Volt to magsafe.

Next is the Current (Amps) it is the consumption. Usually they are controlled by the consumer (Battery in this case). You can not have to much of it since Battery only takes what it needs. The specification for Maximum Amps is for protection of the connector (plug), otherwise to much of it can burn the wires/connectors.

The 85 W charger (20x4.25) are used with 15" and 17" MacBook Pro, so you might be bit on low end with your 60w. It will charge but slower. 

If you are in the wilderness with no plugs and on you Solar, keep an eye from time to time on the Charging.

About this mac > System report > Power. 

There you will see if the battery is charging or been depleted.

Look at he Amperage (mA) number, if it is negative (-) then your battery is been depleted.