One of the first ones is Admiral Preston Cole's victory and possible death at the Battle of Psi Serpentis with Battle group Indie with 13 Cruisers, 23 carriers, seventy nine destroyers, 42 frigates, 5 prowlers, and 50 repair, supply and rescue ships. Cole managed to destroy a good chunk of the covenant's fleet while losing a third of his in the first attack. Later unknown friendlies, possibly Insurrectionists, led a charge from slipspace destroying the Covenant fleet to half its original strength - in the area of 100-150 ships. The unknown friendlies flee quickly and Cole's ship splits off his retreating battle group and taunts every Covenant ship and claims
"From where I sit, we [humans] are the worthy inheritors"
All the Covenant approach his ship where he fires everything he has, then drops 12 nukes into a nearby gas giant, making a momentary sun killing himself, and the Covenant fleet.
The other two battle were the battle of Sigma Octanus IV and Harvest (second battle).