All depends on the team compositions though. But considering you are 3 levels behind, its very likely that the team was doing bad all game anyway. The best way to catch up is to become a team while playing. That way you generaly automaticly catch up by simply becoming more reliable at defending.
And note that since the fort changes, isolating an enemy causes the fort to quickly level the HP value of your target to be reduced to your level (losing 10 armor is often worth 2-3 levels of HP).
Still, at that point its not about kills though, its about surviving. Therefor teamfights are discouraged (but still inevitable). It doesnt matter if a team is 3 levels ahead, 1 combo of your team can still insta delete an enemy. But that only happens with teamwork.
But on 1 thing i do agree,
Being a talent behind should not be a reason to start a fight, since even though you take the initiative. The enemy has the advantage. Even more when they have level 10/20 and you dont. Unless its in about your core, accepting some damage in order to catch up can be a viable tactic. Deaths only set you back further.
But regardless of the level gap, you eventualy will face a fight, better to be prepared for a fight with 3 levels behind, than to not be prepared and lose a player before your team is even together. Soaking can hinder at this part.
Soaking isnt always a good idea to do, the enemy can use that to their advantage aswel and keep ahead. You might catch up with 1 level, if they take down a fort, that fort grants enough XP for them to equalize that for a long time.
Getting a kill is always more beneficial at that gap though, it not only gives you direct XP, after the fight, they also lose a player for soaking. Even trading 2:2 in such fight and losing soaking capabilities is more beneficial since the 2 kills grant a lot more XP than you would with soaking.
Its just that if you fight, you should start that fight with an advantage. Catching a soaking enemy that is out of position is worth getting a group together for, even if you lose some soak. Going 5v5 is generaly not.
Losing 1 lane of soaking, and taking out that hero using 3 heroes yourself, while the enemey was soaking all lanes is worth it. They instantly lose soak on that lane aswel.