Is it possible to connect a 3-wire line without stripping an RJ11 line cord?
North American RJ11 cords can carry two phone lines. Red and Green are the wires normally used, with the outer yellow and black as the secondary. You would only use one pair, and not connect to the other.  

Your phone diagram from here: 



Green is TIP on the RJ11 

Red is RING on the RJ11 

So Red to Red, Green to Green. 

Yellow should be grounded, according to the the diagram above. You can try the ground on an outlet box for the ground. 

It's easier to wire (the question in your title!) if you can get a connection block like this one (preferably without the split plastic boss shown in the photo):