This is the 524th most frequent Icelandic word.
þrjá
The Icelandic word "þrjá" means "three" in English (accusative masculine plural).
In this sentence, 'þrjá' is used as the masculine accusative plural form of the numeral 'three', agreeing with 'hunda' (dogs).
Ég á þrjá hunda.
I have three dogs.
Here, 'þrjá' is also masculine accusative plural, used to count 'bíla' (cars) which is in the same grammatical case.
Við leigðum þrjá bíla fyrir ferðina.
We rented three cars for the trip.
Again, 'þrjá' is the masculine accusative plural, matching 'miða' (tickets) in number, gender, and case.