1) f(x-8) = 6(x-8)^2 = 6(x^2-16x +64) etc if you wanted to expand it
2) f(x)-8 = 6x^2 - 8, Again you can tidy up here if you wish (factor out a 2)
For the last one f(x)= 3 means that whatever value of x you have f is always 3. Plot it - it is a horizontal straight line through y = 3.
f(x+4) = 3...in fact f(x+ anything) = 3!
Notice that there are no x's on the right hand side...so you can't put any value in there - it is independent of x, in other words f is a constant.