So I have 3 independent populations with μ1, μ2, and μ3.

It is given to calculate their average we use L= (μ1+μ2) - μ3

We have random samples for each population with respective sample mean Y1, Y2, and Y3.

Then we get the estimator l= (Y1 + Y2) - Y3

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So the Question is: Is l an unbiased estimator for L?

I know in order to be an UE the expected value of the estimator has to be equal to the actual value, but how do we show this?