Well, at level 10 you should have ... +3 from a magic item (your group should have gotten its first +3 item at level 6, 2 at level 7, 3 at level 8, 4 at level 9, and 5 at level 10. Since the first thing to be upgrades should always be the trifecta of Armor/Weapon/Neck, that basically means by level 10 the group has a total of 15 +3 items at level 10, which by no strange coincidence means everyone has a +3 armor/weapon/neck)
This is wrong on many levels...
First, assuming normal loot distribution, between 6th and 7th level you will find one 10th level item as your highest item found, and it may well be in the adventure/encounter that levels you to 7th assuming the highest level item is in the 'showdown' that gives the most xp. This means that the earliest you can find any +3 at all is 7th level, and you might as well count it as 8th level. This reduces the number of possible +3s by 5-9 there... if you've just reached 10th, then you could have, as a party, 1 13th, 2 12th, and 3 11th items. If you're a nanosecond from 11th, you might have 1 14th, 2 13ths, 3 12ths, and 4 11ths.
Next, you're assuming that the first items that you get are your +3s, which doesn't make sense for a variety of reasons:
First, because even with wishlists loot distribution is not getting the exact single thing you asked for, but the DM selecting from a list. Many groups don't use wishlists and some groups just give out the treasure in the adventure, roll randomly for treasure, or give out the DMG guideline (N+1, N, N-1, gp worth N-1)
Second, you're assuming that all 11th and 12th level items are +3s, when many people are actually looking for items with actual enchantments on them, and thus might be higher in the level span. Many people want Staffs of Ruin and Frost Weapons and Cunning Polearms or whatever... which means they can't afford to 'burn' one treasure pick for an 11th or 12th level +3 weapon when they're waiting for 13th to get the one they want.
Third, there are an awful lot of non-enhancement items out there. Healer's Gloves, for instance, are quite common as 12th level items. Gloves of Ice, Dragonshards, Bracers that add damage to melee basics... even in a land of wishlists, these things really do come up.
Anyhow, that math aside...
At 10th, I think my characters tend to have around...
Lvl +5
Stat +5
Enh +2 or +3
Feat +1
(if melee, +3 prof, if implement +1 accurate)
So, pretty close to their guidelines. I'd suggest that if you're more than 2 behind there, you might be a bit off.