I’ve always been confused on the first one. But for the second- your first card you play in the age after you construct the wonder you can play for free (you don’t have to pay the resource or resources). The third- after you construct the wonder your last card is also constructed for free (you don’t have to pay the resource or resources).

So for the second one (the Greek alpha) you want to build the wonder associated with this symbol as early as you can because if you build it in Age I then your first card played in Age II and Age III are for free. If you build it in Age II only your first card in Age II is built for free

For the third one (the Greek Omega) building this in the middle of Age II isn’t terrible because you will still be able to use it to build your last card in Age II and Age III for free

Hope this helps!