Not Updated For Current Season
This is my first build, so please read it accordingly.
This is how i play Olaf in 3v3, and it works very well for me.
I will write a lot more in this build, just wanted to write down the basics. More about laning and jungling soon.
I use armor penetration marks Greater Mark of Desolation, dodge seals Greater Seal of Evasion, magic resist glyphs Greater Glyph of Magic Resist and flat hp quintessences Greater Quintessence of Health.
As you can see, i build it as follows: 9/21/0. Defense is full, cause olaf must be able to absorb some damage, if you go full offense, you will be vulnerable.
Dodge from defense + dodge runes give you enough to gank effectively.
This is the hardest part, you must adjust your item build to your game.
The main items:
Boots of Swiftness size
Frozen Mallet size
I start with boots + 2hp pot. + 1mana pot.
Your second item should be ruby crystal, more hp and more dmg (<3 Olaf).
Then finish boots, and get phage asap.
Alternative boots --> if you can't dominate your opponents, then you should choose ninja tabi Ninja Tabi or mercury's threads Mercury's Threads (depending on enemies).
After you have your Phage, and you can farm money quickly you should buy giants belt or finish frozen mallet fast. However, if you're being ganked and you can't even afford giants belt, avaric blade Avarice Blade is a good choice (more dmg, and for the time being, faster gold income). After frozen Mallet is completed, youmuu's ghostblade is your next item, it has everything that olaf needs with an amazing active(except HP, that's why you need mallet first).
Another way to go if you cannot dominate your enemy quick is buying Emblem of Valour Targon's Brace, it gives you health regen and life steal, you can get those few much needed lvls and gold in jungle with these, even with hp left to gank a nearby enemy (care to leave enough mana for that).
By this time, in 3v3 most matches are over, but if not, go for one of the following items depending on match situation: Warmogs armor(lots of hp), atma's impaler(against AD enemies + better dmg), Banshee's veil (against magic dmg enemies).