The first build is Olaf more as a damage dealer than as a tank while also being a bit hybrid with Hextech revolver, since if you get CD items with him you spam more spells than almost every champion in LoL per second.
The second build is actually more recommended if you notice that you can't own the enemies, so only to be sure you better get some more survivability while you will also deal nice damage

Greater Mark of Desolaiton:
Armor penetration is important because ingame AMP items combined with your AMP runes and the 10% AMP from masteries deal sick damage
Greater Seal of Resiliance:
Most people play AD Offtanks on 3v3 thats why it's important to have some armor bonus
Greater Glyph of focus:
This will give you about 6% cooldown reduction, if you will just get Ionian boots and another CD item, or at least the CD masteries you will be able to spam with your Undertow almost every second
Greater Quintessence of Desolation:
Since you won't buy really much ad items because if you play Olaf with this build you have not only to find a balance between Tank/AD but between Tank/AD/CD-reduction.

I get 9/21 masteries with him because in both builds I won't get any tanky items before I buy manamune and boots and as you know Manamune is pretty expensive. Also I get a nice bonus on my ghost/exhaust from my offensive spell mastery point and also some AD/AS/AMP.

These are the items that you must buy if you want to follow this build:

First about the item which is most important in this build:
Manamune:
Why Manamune? Well since on 3v3 there is no blue buff or anything like this you sure need lots of mana/mana regen to spam all your skills on the lane. Since it also gives AD bonus from your maximal amount of mana, it is actually a very effective item on Olaf.

Frozen Mallet:
This is actually an item that every Olaf player gets, because of its slow, HP and it also combines very good with his ultimate because while the enemies cant use any CC on you, you instaslow them with every hit.

Last Whisper / Black Cleaver:
Since you don't get pretty much AD items with him it's very important to have at least AMP since most of the 3v3 players are playing their champs as offtanks.

Now the items you don't necessarily need and can replace with other specific items which suit you more because of your team combination / the enemy team:


Hextech Gunblade:
If you want to save some money and not throw it away, than you should better get Blootthirster or Executer's Edge if one of the enemies heal is too damn high

Sunfire Cape:
You can also get Warmogs Armor / Guardians Angel / Hexdringer / Force of Nature as your second tank item. However if it is any of those (expect Warmogs) then you better not get Atmas, but instead Ghostblade / Infinity Edge / Bloodthirster or if you feel that you are still not tanky enough, Warmogs Armor / Guardians Angel / Hexdringer / Force of Nature

Other items you may like to buy are

- Trinity Force

- Wits End

- Madreds Bloodrazor

- Nashors Tooth (Not really effective tho)

Of course I max my Q first because since I buy manamune first I depend on my Q spam. After that I max my W because I dont get any lifesteal items until end game, so I need some healing in the teamfight. At last I max my E because nothing is left. And of course I skill my ultimate on the levels 6, 11, 16

I pick ghost / exhaust because these are the best spells for olaf

Ghost: You get some nice movement speed, + you Q slow, + your Frozen Mallet slow, (+ the MS bonus from the black buff, + the slow from the red buff).

Exhaust is also very usefull since you normally take the top lane where you often face another AD bruiser and normally without exhaust it is way harder to beat him at 1 vs 1

Pros:

-High mana amount
-Good balanced between Utility/Tank/AD
-Very strong laning
-Good ganks
-Still good healing

Cons:
-Needs skill to play since you spam your skills very fast in teamfights and have normally 1-2 items you can activate in your inventory
-High gold costs for Manamune
-No early healing item
-You need to land every axe right there where you will take it as soon as possible to reach the maximum effectivity of this build
-Not as tanky/ AD as in other builds

If you feel unsafe or know that you will get ganked if you would stay at your lane, you better go into the junngle and kill some creeps. Since you have a high mana amount and get healed by your W you lose almost no HP/Mana and return safely to yuor lane

Dont fear to push your lane. Since you spam with your Q anyway, you may as good make it a bit faster by also autoattacking. However it is very important tho, to ward the mid jungle, to know when you will get ganked.