No matter how many times you heal your opponent's minion, you may only add 3 to 
Hrethel's strength (that's what "limit +3" means)
What does 
Larder have to do with adding to 
Hrethel's strength?
Just for fun, I'll run through a little scenario:
Hrethel vs 
Shelob, Eater of LightOn table: 
Web x4 (each with 3 minions stacked), 
Larder x4 (each with 2 tokens)
Hrethel is bearing 
Swift Steed.
FP Player assigns 
Shelob EoL to skirmish 
Hrethel.
Skirmish begins. (
Hrethel 6 Strength vs Shelob 44 Strength (+3 str for each minion stacked on a Gollum possession: 12 x 3 = 36 + base strength 8 = 44)
At the start of the skirmish, Shelob exerts due to 
Swift Steed.
FP Player has first action: Uses 
Hrethel's text to heal Shelob to add 3 to his strength. (9 vs 44)
SP has next action: Uses 
Larder's text, discarding a token to make Shelob str +3. (9 vs 47)
FP has no more actions.
SP continues using each 
larder's text, removing all tokens to add 21 more strength to Shelob. (9 vs 68)
FP Player passes.
SP passes.
Hrethel is overwhelmed and killed. Skirmish ends.