This is the 208th most frequent Czech word.
třetí
"Third"
In this sentence, 'třetí' is used as an ordinal number indicating the sequence of days.
Třetí den jsme se vydali na túru.
On the third day, we went on a hike.
Here, 'třetí' serves to indicate the number of times an event (visiting the museum) occurred in sequence.
Byla to moje třetí návštěva v tomto muzeu.
It was my third visit to this museum.
In this case, 'třetí' identifies a specific option by its order in a list.