I have a 3 in 1 pear tree that i planted about 7 years ago... past few years the tree has produced pears... but over the past few years the One grafted branch produces nice size pears... but the other two grafts produced 100 small marble size pears that never got bigger.... can someone please tell me why this is? And what I can do to fix it? I have tried removing the clusters of 4-5 down to 1 or 2 pears...

I believe you only have one graft that is good and the other two are just “wild” pears from the root stock.

Are the leaves on those grafts also sub-par? If so, my guess is that it is a graft compatibility problem. Nothing can be done about it but leave them on there anyway to provide pollen for the good graft. I note that you have tried thinning the fruit and didn't get much result.

I like OKforesters take. Go with that!

Do the two graft branches with the small pears have pears that look alike, or are they obviously two other varieties? If they are two varieties, at least one of them is not a rootstock pear variety. 200 pears on two branches is much too many. Severely thin them early, like when they are cherry size. Leave only one pear per bunch . It might help to put compost or fertilizer watered in around the tree in winter too. If you thin to about 40 total on those two branches, fertilize, and get adequate rainfall during the season the pears should be considerably bigger. If they aren't, most likely they will not be on those two grafts. You could top work the tree and graft new scions on those two branches with scionwood from the large pear part of the tree. After they are well established, cut the miniature pear grafts off.

The pears on the two different branches look identical... even the leaves (which are healthy) .. just the one branch/ graft does good! So most likely it’s just two branches from a mini pear and one graft from a edible pear?

Most likely, like OKF says, only one graft was successful and the other branches are from the rootstock. Rootstocks are selected for vigor, hardiness, disease resistance, etc., not fruit quality.

My old pear tree, 20+ years was a dwarf, I planted it too deep and it became a full sized tree, half my pears fall off before ripe, skin on pear is tough until ripe, I'm starting to get a fungus problem in pear tree and grapevine, thinking I better oil them down before leaves come out in spring
My new pear tree, had three pears this year, perfect tasting and texture
I've heard the multiple fruit trees usually don't do well, but I never tried one