2020-11-10 : Sparktongue Dragon's triggered ability goes on the stack without a target. While that ability is resolving, you may pay {2}{R}. If you do, a second ability triggers and you pick a target that will be dealt damage. This is different from abilities that say "If you do . . ." in that players may cast spells and activate abilities after mana is paid but before damage is dealt.

2020-11-10 : While resolving the triggered ability of Sparktongue Dragon, you can't pay {2}{R} multiple times to have it deal more than 3 damage.