I personally really dislike when a story jumps from third to first.
Why?
Because the first person POV colours the story by being limited to what the character knows or believes. The entire world is shown to the reader through the eyes of one individual. That said, the character still doesn't have to be reliable at all.
A strictly limited third person narrative can achieve this as well if we only follow one person. The distance to the character grows a little, but there are stories where this kind of detatchment is perfect.
The usual style for making the reader know more than each character is a limited third person multiple point of view. It's considered good form only to switch POV with a clear and distinct break. It's perfect for quickly ramping up conflict based on misunderstandings.
Lastly you can go ninteenth century on your readers and run with god-mode. I feel that this is sloppy writing, but admittedly there are a large number of classics doing exactly this. It's also the most usual version of third person POV found here. I suspect an influence from cartoons (manga, manhwa, etc) where you can have multiple thought bubbles inside one frame. Poorly done this kind of POV will degenerate into an orgy of tell and very little show.
Anyway, in short. Pick FP or TP and stick to one of them. Sure, feel free to run your frame story with an FP narrator finding a room full of old letters and then offer your story of what happened fifty years earlier in TP based on the contents of those letters.