When they get too big i will give them for three more smaller ones.His gain My loss.
I'm assuming you mean wherever you are getting the fish from - good that you have a plan. I wonder though - why get koi when you know they are unsuited to your size pond? Those koi will be too big for your pond after a single season. Why not get goldfish or shubunkins that can grow and stay in your pond? You (and your grandchildren) will get attached to your fish and it's fun to see them as they mature from year to year. Most people get koi for the colors, but you can get very colorful shubunkins (check out some of @addy1 's posts of the lovely shubbies in her pond) and goldfish are every bit as friendly as koi.
Just sharing an alternative idea - but your pond, your decision.