Confirmed way to get Reznov and Mason!

Reznov: Find the Ushanka Hat (on the spire in Estates,the fountain in Estates or the fountain in Asylum

• Kill an enemy at extreme range
• Place Top 5 in the match
• Finish the match with the Ushanka hat in your inventory

Mason: Find Masons White Tee

• Play with the Reznov skin equipped
• Activate sleeper agent (supposedly the Array intercom)
• Place Top 2
• Finish with Masons White Tee in your inventory

(Thanks to u/YouthWasted in the comments)

Menendez: Get the Locket from a supply drop

• Get a Shotgun Kill
• Get a Melee Kill
• Finish the match with the locket in your inventory (Possible placement/win requirement)

(Thanks to my friend u/MDridge7 on Xbox)