It doesn’t automatically invalidate the appointment as executor, but he could renounce it, or if he can’t, or won’t, the two other executors could either just apply and get a grant of probate with “power reserved” to him to apply if he ever wanted to, or if they want to make completely sure, they can apply to the probate registry under s116 of the Senior Courts Act 1981 to have a grant of probate made to them, excluding him as someone obviously unfit to act as such.