Above three cases are all different 다's usages.
1. 좋지만 vs 좋다지만
As javiskefka said, 좋다지만 is short for 좋다고 하지만.
And it refers to the speaker's opinion with the common sense.
2. 음식점 찾아다는거 vs 음식점 찾는거
You might wanted to say 음식점 찾았다는 거 or 음식점 찾는다는 거.
This 다 is just an ending of the sentence.
음식점 찾았다 + 는 것 : The thing is + I found the restaurant
음식점 찾는다 + 는 것 : The thing is + I find the restaurant
음식점 찾는 거 : Finding the restaurant
3. 태워줄께 vs 태워다 줄께
This 다 is short for ㅏ/ㅓ다가.
The construction of this sentence would be
[Verb A] + ㅏ/ㅓ다가 [Verb B]
means "To [verb B] with the action of to [Verb A]"
태워다 줄께 -> 태워다가 가 줄께 : I'll pick you up and bring you there.
태워줄께 : I'll let you in this ride.
이것 좀 갖다 드세요. -> 이것 좀 가져다가 드세요. : Bring this and try(eat) some, please.
이것 좀 드세요. : Try this, please.
그는 열심히 돈 벌어다 (그녀한테) 다 바친다.
He earn his money so hard and give it all to her.
I'm wondering if I explained well.
Hope this helps, anyway.