About "weighted average".
Probably this is it:
Let's say, that you have two inputs: x and y (x, y are numbers).
We need two other numbers (the "weights"), let them are a and b.
Weighted average of x,y is the next: (a*x + b*y)/(a+b)
The normal average is (x+y)/2, where the x and y are equally "important".
With weighted average, you can make one of x and y more "stronger" than the other. The stronger input will influence the output more.
Now back to your question:
You have three inputs: IR, E1 and E2 encoders.
Let us say, that you counted the headings from the inputs : hIR, hE1,hE2, for example:
hIR=60, hE1=50, hE2=52 (grads).
The average will be (60+50+52)/3=54, so your robot's head is in 54 grad.
But, you experimented, that the encoders are not as good as you planned, so you should make the IR inputs stronger:
Now your problem is, which numbers should you use as weights.