Assuming it's a Honeywell 22mm 3 port .. or similar brand like Landis ..
There is a seal in the centre of the valve paddle outlet that can leak .. on the top underneath the motor assy. If only three years old, you could just replace the valve assy. Some 22mm valves here ...
(buy same brand as valve) Most valves same tho bar a few exceptions. If you have to buy a new valve complete .. and it's 22mm .. Honeywell V4073A. Will mean a partial drain-down tho.
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