HH is right, pH 6.5 is close to neutral, and is well within tolerances for most vegetables and other plants.
As for sampling soil amended with 3 different topsoils, you can do that if for some reason you want to compare those areas (you are noticing crop growth different in the 3 areas for example). Just remember, you should always collect soil as a composite of multiple small samples across the root zone. For each area you want to test, take a shovel slice the depth of the root zone, collect a narrow vertical slice from that (about 1 inch wide), do the same at 6 or so random locations (more is better). Mix all subsamples thoroughly in a plastic bucket, take your sample for testing from that.