No. You need to first get through check in. For example, Budapest airport is annoyingly slow while Vancouver is surprisingly fast. Then there is security. The waiting time for this varies from day to day and hour to hour and often terminal to terminal. Some airports require you to
Remember, you will still need to pass through security at least 60 minutes before departure and be at the boarding gate no later than 30 minutes before departure
so you need to clear security well before boarding.
At some airports and some flights you also need to clear passport control. You might have exit controls, you might be entering Schengen, you might have preclearance to the USA and so forth.
And, of course, boarding opens earlier for bigger planes than smaller ones.
Three hour is almost always an overkill but if you get secondary screening and late from the plane, the airline can say "you should've been here three hours earlier, we told you". Sometimes they won't (thanks Alaska!) but it's there.
Also, the airlines will give a time that works for everyone. For example, I have made my USA bound flight several times at Vancouver airport with only 60 minutes but I have a NEXUS card and I am a very experienced traveller -- so I get a special queue for security, I am packed to breeze through security, I go through the border via a machine and no queue. It is neigh impossible for an ordinary traveller to get from airport door to gate in <30 minutes as I do.