How to Repair Damage to Tree
I have a fairly large tree with three large branches splitting from the stump. Rainwater gathers in the intersection of the branches. How or what can I do to divert the water without harming the tree? Could I put cement in there to divert the water?
This article may help you. https://www.gardeningknowhow.com/ornamental/trees/tgen/patching-tree-hole.htm
You might want to consider hiring an arborist for advice.