It's quite a while since I played F3, I've been doing New Vegas but I thought I'd try coming back to F3 so I could compare the two games.
First off out of the vault it seems a lot easier than I used to find it. I went for a wander around and ran into 6 raiders, killed them all without much trouble, using a couple of stim packs only. I also found it strange that their camp had no loot in it at all... no spare ammo, not even a bedroll for them to sleep on.

So I've wandered to Megaton, done a couple of quests and been given a house already.

So I'm wondering why I'm finding it so easy? Maybe it's just that I'm a more experienced player overall, as F3 was my first venture into the Fallout world and I found it very scary first time round. Or maybe it's my time in New Vegas, and F3 is the easier game?

Did you play on hardcoe Mode in New Vegas?

Other than that I suppose it's cause in Fallout 3 you're showered with ammo and chems whereas in New Vegas it's a bit more limited.
And Fallout 3 also uses Damage Resistance instead of New Vegas' Damage Threshold:
DR:
You have 100 in health, you have 20DR, an enemy shoots a bullet with 20 damage, 20% is reduced, making it 16 in damage, you are hurt for 16 points.
DT
You have 100 in health, you have 20DT, an enemy shoots a bullet with 30 damage, 20points are reduced, making it 10 in damage, you are hurt for 10 points.

Same for enemies.

In New Vegas DT has a certain minimum damage that has to go through, I think it's 10% or something.
In Fallout 3 higher DR is 85%, but enemies never has that.

Experience definitely made it exponentially easier for me. I'm sure that's a big contributing factor.

Super mutants & nightkin still use DR in NV, as well as certain other characters, and the Courier potentially.

As far as damage goes, a VATS strike in FO3 gave you, like, somewhere around 90% damage reduction during the animation whereas NV only gives you a 25% reduction. The crit chance in VATS is also significantly lowered.