In an '80s Canadian house I took out a wall switch and now I can't figure out how to rewire it.

The box has three cables coming into it. They all come off the same panel breaker. Two of the cables have a white, a red, and a black wire. One has a white and a black. All of the white wires are joined together. So I have three loose black wires and two loose red wires. I'll call them B1, B2, B3, R1, and R2. I tested joining them and found the following:

* B1+R1 = ceiling lamp which I want to switch
* B1+R2 = a wall socket
* B1+B2 = two other wall sockets
* I don't know what the third black wire does.

So I guess I need to twist together B1+R2+B2 on one side of the single pole switch and R1 on the other side. And I guess just terminate the unknown black wire with a twist-on wire connector? 

Questions:

* Is the above the way to go?
* I don't like trying to twist three solid wires together and attach them to one pole. It seems very insecure. Is there a switch that has multiple screws on one side?
* How would I find out what the odd black wire is for?