hunters mark isn't worth its bonus action compared to just shooting with crossbow expert.

Actually, as long as you have Extra Attack, Hunter's Mark will always deal more damage than a bonus action attack.

Let's say you have a fight that only lasts one round, and you're using a heavy crossbow. Your options are:

1. Hunter's Mark & Two Attacks, which deals 2d10 + 2d6 ~= 18 damage
2. Two Attacks & BA Attack, which deals 3d10 ~= 16.5 damage

For every extra round in the fight, the damage of Hunter's Mark dramatically increases. In an eight-round boss fight, for example, Hunter's Mark ends up dealing over 60 more damage, assuming you don't lose concentration.

The only time the third attack would be better is if you need to kill three targets who can all be one-shot with or without Hunter's Mark.