Q1: When my opponent has a Digimon with 8000 DP or less, can I activate this card's effect and have this Digimon get 3000 DP but choose not to delete my opponent’s Digimon?

No. If there's a valid target, you must choose it when you activate the effect. You can't choose to not activate the effect when there is a valid target.

Q2: My opponent has a Digimon with 8000 DP or less and an effect that reads, “This Digimon can't be deleted by your opponent’s effects,” and another Digimon with 8000 DP or less but no effect. Can I intentionally have this card’s effect target the Digimon with the effect preventing deletion and have this Digimon get 3000 DP?

Yes. You can choose your opponent’s Digimon with 8000 DP or less as a target, even if the chosen Digimon has an effect that prevents it from being deleted, in order to get the “if this effect didn't delete” part of the effect.

Q3: The memory is at 0, I use this Digimon with ST7-05 Growlmon in its digivolution cards to attack, and it deleted an opponent's Digimon with Retaliation in battle. Is this Digimon deleted by the Retaliation effect at such times?

Yes, it's deleted. When your opponent's Digimon is deleted in battle, it causes ST7-05 Growlmon's inherited effect and Retaliation on the opponent's Digimon to trigger simultaneously. The turn player's effect activates first, the memory becomes 1, and this Digimon's Your Turn effect can no longer activate, therefore it's deleted by Retaliation.