[scooble]

Assuming that this is not already covered by your contract, and that you are not going to flatly refuse, it may be worth thinking about possible areas of negotiation.

For example, if you have to be available for 3h per week, you might suggest retaining 1 vacation day of that week. Similarly, you could suggest that you retain the vacation day if you end up working more than one hour on any given day of your vacation.

As well as ensuring you aren't giving away your vacation time for free, this also sets boundaries. The risk here is that your 3h a week effectively turns into you being on call for the vacation.