when i get my 3rd gear do i have to spend frags

Yes — for Third Gear in Blox Fruits, you generally have to spend fragments to unlock the upgrade after completing the trial. A guide found for the Race V4 gears shows the Third Gear step requiring about 2.5K fragments after the needed training/trial progression.

What that means

• You do the required trial/training first.
• Then you pay fragments to unlock the gear upgrade.
• The fragment cost appears to increase with later gear steps.

Useful note

A separate Blox Fruits wiki page confirms that gears are part of Race Awakening progression, and fragments are a core currency in the Third Sea used for upgrades and related purchases.

Bottom line

So the short answer is: yes, Third Gear usually costs frags.

Would you like the fragment cost for the other gears too?