Sorry I didn't notice this before. All 3 of these players are coming off of prospect contracts which means they are eligible for discounts. I just want to lay out your options here.
Jarnkrok -
Normal extension $3.5 for 3 or 4 years
Discount extension $2.5 for 5 years
Dumba -
Normal extension $3.6 for 3 or 4 years
Discount extension $2.5 for 5 years
Zadorov -
Normal extension $0.9 for 1 or 2 years
Discount extension $2.0 for 5 years
A Prospect Discount extension must always be for 5 years and the minimum value is $2.0M. As you can see with Zadorov, you'd have to actually pay him more since his normal extension value is less than $2.0M - so that one may not make sense. But the other 2, particularly Dumba, would make a lot of sense. You're tied up for more years, but at a better price. And since they're both young, the length isn't a huge concern.
Let me know what you want to do on each player.