Heroism: A persisting spell?

Heroism

Level 1 Enchantment (Bard, Paladin)

Casting Time: Action

Range: Touch

Components: V, S

Duration:Concentration, up to 1 minute

A willing creature you touch is imbued with bravery. Until the spell ends, the creature is immune to the Frightened condition and gains Temporary Hit Points equal to your spellcasting ability modifier at the start of each of its turns.

Using a Higher-Level Spell Slot. You can target one additional creature for each spell slot level above 1.

Until the spell ends, you gain THP at the start of your turn.  Obviously, immunity to frightended would end because the spell ends.  But, it doesn't say you have that a set amount of THP, it says you GAIN them while the spell is active. 

When the spell ends, do the THP persist?  Could one technically, with a +3 modifier, just cast Heroism and have 30 thp at the end?  What is to stop a player from just stacking this using 3 spell slots and have 90thp at lvl 2?

Duration

Temporary Hit Points last until they’re depleted or you finish a Long Rest (see the Rules Glossary).

They persist

Also just read the rules for THP.  

They don't stack.  So you get to have 3 per round, not 3,6,9. 

The relevant rule:

Temporary Hit Points Don’t Stack

Temporary Hit Points can’t be added together. If you have Temporary Hit Points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 Temporary Hit Points when you already have 10, you can have 12 or 10, not 22.