Vikings for the most part are not very useful for early to mid game team fights since they only can auto attack until level 7. Try to soak xp as much as possible in every lane. During team fights only bring Olaf so he can somewhat tank for the team. Since most players try to pick off vikings first it could give your team a few seconds to deal massive damage before the fight starts getting scary. Your team may lose the first couple of team objectives however if the enemy team leaves the vikings alone your team can get ahead in experience so team fights will gradually become easier as long as your team isn't too reckless and that having multiple allies dying will negate your experience gains from your vikings.

Level 16 Execution is the most important talent in this build. Focus fire on a weakened enemy hero and your can probably kill it off within the 3 seconds that the bonus damage is active. Olaf can charge at an enemy causing that enemy to slow down a bit which allows for the massive auto attack damage burst.

Level 10 Long Boat is important because with this build you're meant to get into people's faces and this ultimate may be a game changer when used on an enemy team to punish them for "over extending" especially with a weaken team.

Level 20 can either be Ragnarok 'n' Roll if the team either still needs to secure objectives/ when team fights are still plentiful or Viking Fury of the Storm to make their melee damage insane with the Executioner and Nordic Attack Squad combo.

Nordic attack squad is very nice in this build to accelerate damage dealt especially to heroes that are somewhat tanky.

Mercenary Lord is a decent talent because it affects all three of your vikings so they take 50% less damage from minions and mercenaries but they can increase damage output of nearby mercenaries allows you to push a lane hard after your team takes out a camp or two in the same lane. I only say "decent" because on average I find it useful maybe at least once or twice a game and sometimes that is enough to takedown a few towers.

Vikings Bribery is decent when taking out siege camps with only Erik but I find myself using it maybe once or twice on average. You can swap this for spy games to make Erik invisible so he can soak xp if minions die nearby or to capture a shrine and if the enemy team has no basic AOE abilities them they can't capture a shrine.

At level 20 you will need to keep your vikings together in order to use the long boat but also to deal auto attack massive damage.