In tier 3 lockdown one small heart is just not enough; Here's my suggestion:
Tier 1 Lockdown (Stays the same): when you defeat an opponent they drop a small heart which heals one bar.
Tier 2 Lockdown: when you defeat an opponent they drop a medium heart which heals 2 bars.
Tier 3 Lockdown: when you defeat an opponent they drop a large heart which heals 3 bars.
Remove hearts entirely. Leave healing to guardians.