I can write first-person great, but many stories can only be told third-person and i dont have much experience here. It always feels clunky, doesn't flow right. And get distracted by pronouns and what is a private thought of the character and what is just descriptive of the situation. I cant really find my voice in 3rd person, how should i fix that?
I have the opposite problem. I can write in third person fine and much prefer this but struggle with first person.
My solution? Practise.
To give you an example, I wrote a story of two women having an argument in a cafe from the third person perspective of the waitress serving them. Obviously, she couldn't hear everything that was being said and could only go on body language and brief snippets of conversation.
Then I re-wrote the story from both women in the argument in first person, really making sure I got into their head. Then I went back to third and was able to flesh out things I hadn't thought of before in my original draft and later, allowed me to put them in without having to go 3PP.
So I'd suggest you try this. Write it in first person, perhaps from two or three characters POV. Then write it again from the third person view. Obviously, you can't say what the characters are thinking (though that doesn't mean you can't put in parts like "Tom paused as he appeared to think, his brow furrowing") but you can add extra details in appearance.
And I notice your comment about plot structure of the third person. Plot is plot. Yes, some stories are better in third and some would never work in first (can you imagine A Song of Ice and Fire in FPP?) but To Kill a Mockingbird is all the more powerful because of it's FP perspective. It's all about telling your story in the best way possible to make it a better read for your readers.
Pick three or four third person scenes that you like from other writers. Type about 500 words of each into your word processor, using whatever format you normally use for manuscripts.
As you type (and afterward) notice the rhythms those authors use, and how they make it clear whether the character is observing something (through the five senses) or thinking something.
I like @Stephen's idea, which I think you should adapt into a series of exercises.
Sit in a park or at a café somewhere and people-watch. Try to write down what you see. You can't know what people are thinking; you can only observe. So write that down: She spoke. He laughed. The dog barked. The waiter looked bored/interested/tired. She kissed the first woman on the lips and hugged the second woman. The little boy whined that he wanted more ketchup.
Once you're used to writing what you see, take that home and try to use it as a skeleton for a short piece. What could they be talking about that made him laugh? Why was the waiter tired? and so on. It doesn't have to be a story; just practice figuring out motivations from outside observations.
You can do this with TV too, but I would turn the sound off so you aren't cheating by hearing the dialogue. The advantage is that you can pause and rewind to study faces and gestures.
I prefer to think of an either "almighty character" or a kind of Carnival Barker when writing in the Third Person...he can do anything he wants...and indeed he should passing through time and space, logic and reason...as mere impediments to give a wonderful insight into your story for the readers that the characters you have created have no idea (nor could they) that he is even there. If done well you can still surprise your reader too with the characters themselves...we are human after all and humans do odd things...and to me this is the art of creating a great story and hopefully a good read.