If my Sorcerer has used up all of her level 3 spell slots, but wants to use a reaction to cast Counterspell, can she use 5 sorcery points during a reaction to regain a level 3 spell slot to cast the Counterspell?
Creating a spell slot is a bonus action and thus has to be done on your turn.
Creating Spell Slots. You can transform unexpended sorcery points into one spell slot as a bonus action on your turn. The Creating Spell Slots table shows the cost of creating a spell slot of a given level. You can create spell slots no higher in level than 5th. (PHB 101)