There are two parts to consider. One is the appreciation in your home. The other is the amount leftover on the loan when you sell. The difference is called equity. After that, you will need to consider the cost to sell a home, which is typically 10%. For your particular case:
Appreciation assuming 2% over the next few years which is very conservative but inline with historical norms: 135k * (1+.02)^3 = 143k. But since you are assuming the home will sell for 150k we will just go with your number.
As for the loan, lets assume your initial payments go towards the interest on the loan (which in practice is likely true). I'm assuming you have a fixed rate mortgage with a 30 year term. Since you paid 20k down already, the amount leftover will be approximately 115k.
So the equity you have is 150k-115k = 35k. If you include all the costs to sell the home (realtor fees, taxes, etc), your looking at ~15k to pay out. 35k-15k = 20k.
So you will gain about 20k if you sell the home in 3 years, assuming you sell it for 150k and the loan you received is fixed for 30 years.
Of course we failed to include the amount you save in taxes every year while owning the home. But the above analysis is a fairly good approximation.... more