I got called a noob by my team for not being able to hold out against 3 champs.... I play Fiddlesticks, he isn't a tank, so how am I a noob? lol. People are stupid some times. I find that people think casters are supposed to hold out like that, even though they don't have the health or armor for it. I don't understand it Reading your post, it seems like a tower was probably being pushed or something. You should have simply let the tower fall and ran away. If it was a point where there were three of them and your team wasn't coming (or would be too late), it's best to let the tower fall instead of dying along with that. Sometimes it's better to do counter-intuitive things like running into the enemy tower if it means your team's carry or two teammates are able to survive or to let the tower just fall instead of trying to protect it.
Of course, if this wasn't the case, meh.
However, I've heard people say Tier 1 runes are worth the VERY cheap IP (but not Tier 2) they cost and can serve you well at below-level 20's and fill in space once you reach level 20 while you're still in the process of getting your Tier 2 runes. Personally, I didn't do this, but it's a viable option. If you're willing to hold out, though, you'll actually see that you can buy maybe one or two Tier 3 runes with the IP spent on a full page of Tier 1 runes. If you're into it, I'd recommend Tier 1 runes, but it's perfectly fine to not get them.
Spoiler for Tier 3 Runes to get :
If you feel like reading this, it's a VERY strong recommendation for what tier 3 runes you should at least start out with.
Flat Health Quints are pretty dang nice. They work for casters, tanks, and AD carries; 100 extra health to start with is simply amazing.
For marks, you're going to want magic/armor penetration. You'll pretty much use them for everyone but tanks (but even some tanks use them).
For seals, Mana Regen per Level are amazing. If you ever find yourself running out of mana often but don't want to get mana carries (such as Manamune; this occurs often with many AD carries), this is the answer to your prayers
For glyphs, CDR (flat or per level) are pretty nice as well as magic resist (flat or per level).