Can't speak for anyone else, but from my own personal perspective....
I've already had two. I don't believe for one second those two are all of a sudden ineffective after a few months. Less effective? Maybe so. Ineffective? No chance.
I don't get colds, ever. Not since I was kid at least. I've never had the flu in my life. Wifey worked in a pre-school for a decade and eventually left because they never stopped sick kids coming in and wifey picked up literally everything. NEVER ONCE caught anything off her despite living and sleeping with her every single day and night for those 10 years. Brother, sis, and mother catch plenty and can take months to shake it off. Never caught anything from either.
And I actually like to avoid viruses. ALL viruses. Having a chimp's Adenovirus filled with viral DNA injected in to me every few months when I've already had it twice just seems a bit - meh.
And I'm an unsociable individual in real life so don't see many people. And those I do will get swatted and kicked in the shins if they get too close. And I'm anal about personal hygiene.
So, 3rd jab? Not a slam dunk by any means me 'ol chum.