Well known problem - do a search for "blend motors", likely to be one of them. Expensive at a dealer (over £1000 if you can find one who knows how to do it!!) but there are indys who will replace all three using a "workaround" for around £300. Some indy's will splice in a single blend motor for less. Forgot to mention, if you have the fault codes read by someone with a Faultmate, it will show up the actual blend that is playing up.