Is it common to have too few resources for most cards in Age 3 of "7 Wonders"?
First off, Seven Wonders is meant to be a 7-player game!  You got the "normal experience".  It's not as if resources are much less scarce in the 3-player game - you do remove more than half the cards from the mix in that game, after all...

I've never found the Age 3 cards to be "unaffordable", so yes, I would suggest that if you find yourself unable to take actions other than "discard for 3 coins" in Age 3, you're probably playing suboptimally to some degree.

Having said that, when I say I can "afford" an Age III card, I don't usually mean "pay the exact resources for" it.  I mean one of the following:


be able to pay for most of it, but have a big stockpile of cash to buy the extra resources I need from my neighbours.
use yellow cards providing "one free resource of any kind" to plug the gaps in my production.
use my Wonder's special power to get free resources or builds, if applicable.
keep a close eye on "upgrade chains" of cards from Age I that get me free cards from Age II that then get me free cards from Age III.


So in many cases I'm shortcutting cards into play instead of actually paying for them.  But I definitely think that if, more than once or twice in Age III, you're looking at your hand and thinking "there's nothing here I can play, at all", something has gone badly wrong with your strategy!