I live in Carlsbad ca zone 10a. Last season I planted three zonal geranium plants in a long rectangular pot. Although plants are growing but these already look quite over crowded. I am thinking of taking out the whole block and separate out the plants by cutting through the root ball for making three individual plants for re-potting.
My question is: will cutting through root ball kill the plants or these will be fine and continue to grow in the new pots.
Any suggestions from learned community members please.

As a rule, when separating plants, I prefer to tease the roots apart, which may be easier if they're soaked first.
But I don't see any problem with trimming the root balls. As geraniums will readily propagate from stem cuttings, they should have no trouble regrowing a few roots.

You can divide them in summer if you must, but you might find that cutting the tops back hard when you divide them will help keep the entire plant from collapsing due to the water demands of the top of the plant outstripping the ability of roots to deliver.

If you do divide soon, make sure you keep the roots wet for the entire time you're working on them. Fine roots can dry out and die within minutes if you don't keep them soaked while you're doing the rootwork.

Spring is the time to divide geraniums left in a dry pot over winter.

Al