3 months notice period is too difficult to serve
It's not uncommon for 3mth notice periods to be in place for upper management, the assumption is that 1mth notice is insufficient for handover and hiring of a replacement, especially if the departee needs also to clear leave.
Not uncommon also, is if a company enforces the 3mth notice even if they ask you to stop work. I've had friends who had to discharge all their responsibilities and access to company network/property within a week, but had to serve out the notice - this is not uncommon for those working with proprietary information.
As for employers - they are aware of the duration of notice for the type of positions they are seeking to fill. If your peers generally only require to serve 1mth notice, then yes - you are disadvantaged. If not, then potential employers know to expect to wait, or to buy-out potential new hires if time is of the essence.
At least that's my experience so far.