parallel octaves or unisons, for what strict counterpoint is, just "kill" voices, that is..
if two playes on the same instrument or on different ones, play the same notes in paralle 8th or unison, it is pratically just one voice that you hear, even if there are 2 instruments, and instead, if one uses other intervalls that are consonants and musically more intresting like 3rds or 6ths then you hear the two parts that play good together and that means more voices heard...
not that octave or unisons cant be applied, of course they help define a melodic line better, and also reinforce it.. but if u have just 3 instruments, and 2 of them do just octaves, then what u hear is basically 2 voices... that's what i meant.
but anyway octaves or unisons are better sounding than 4ths and 5ths because those are like octaves...
...in the sense that they are resting places, where the harmony is so consonant that you ear doesnt have much stimulation, so going for parallel 5th or 4ths doesnt give much, since they just go from a resting place to another, and u just get nothing intresting musically... so better the 3ds and 6ths that have this particular character that makes them more intresting to hear than 5ths and 4ths, even if u have some them here and there...
hope i made sense here again.
to go back to your piece... for what concernes it, well, i think there's nothing bad... you just doubled other instrumetns to give a different nuance and to give more importance to the themes. nothing wrong with that.