Lord_Johnny:
Now, while I'm not saying you're wrong, but a CR 1/3 mean 3 creatures are a fight for a party of four? I've NEVER heard that rating before.
That is the literal meaning of CR.
A monster of CR "N" is an average fight for a party of 4 level "N" characters. As characters can't be less than 1st level, fractional CRs add up. So yes, 3 level 1/3 monsters are an average fight for 4 level 1 characters.
What does "average" mean? That's a little more vague. I don't have my DMG in front of mean, but one such fight is supposed to cost...one fourth? one fifth? something like that of the party's "resources". (HP, spell slots, limited feats, magic items, etc.) Such that if they have five such fights in a row without rest, they have to go rest before having another fight or be screwed.
Or more generally, the fight is not hard but there is SOME risk. For example, one lucky crit by a goblin can take a character into the negatives. Heck, even a regular hit might take out a Wizard. And note that the goblins have bonuses to Hide and Move Silently, they have javelins and they are cowards. Three goblins ambushing the party and flinging javelins can take out someone just in the surprise round. (Ambush doesn't add to the CR/EL.) If even a level 2 party met three goblins, the risk would be much smaller. And there is absolutely zero threat against high level characters, which is why a party of level 9 and up would get 0 xp from this fight. (If the EL is 8 lower than the average level, you get nothing.)
Yes, I'm aware that a single Sleep spell wins this combat and every party with a Wizard will have it. That's not part of the calculation. CRs are calculated for suboptimal (but still decent) parties.
I think every level the DM's supposed to give them one hard fight, a couple of easy fights and a couple of level-appropriate fights. Or something. Haven't looked at the DMG for a long time.