I do that many for several reasons. But first Ishould point out I do UP to 3 a week not always three.
I use to do all my changes on Friday afternoon, but life kinda got busy that day so I started doing them other days just randomly when I had a chance, and insteed of trying to remember if I had done one recently I just did it. So thats one reason why.
I also got frustrated at the dirty plecos making a mess in the water colum before I added poret into the sump, so I figured that the easiest way of removing floating particals was to take out the water and replace it with partical free water. With the poet the water is significantly cleaner visualy, but I guess old habits die hard.
I don't pay for water which is probably another massive factor into why I do so many, if Idid I am sure I wouldn't do anywhere near as many.
And lastly it takes about 2 minutes effort on my behalf to put a hose in, switch of a tank, suck the end of the hose and let it drain. Then remove said hose with a differnt hose and turn that one on, remove once full then switch tank back on.
So that is my rational behind my water changes