I'm a little uncertain what to add to my attack rolls. Can anyone confirm which of the below highlighted modifiers I should be adding to my D20 roll for attack with the longbow? I am proficient in archery. Do I add all three or just some?
The +7 that D&D Beyond shows next to your Longbow attack is what you should add to your attack roll.
D&D Beyond adds the +3 Dexterity modifier that you circled, the +2 Proficiency Bonus that you circled, and an additional +2 (probably from Archery fighting style) to come up with the total +7.
The two upper values you've circled both factor into the lower value that you've circled, which is the correct one.
In general, attack bonus = relevant ability modifier + proficiency bonus (if proficient) + other bonuses (if applicable).
In this case you have a +3 Dex modifier and you're proficient with bows (another +2). The value you've circled in the "Actions" tab is +7, so you're getting +2 from "other bonuses". In this case, you've picked "Archery" as your Fighting Style, which provides the final +2.