Agfdhyk

I am writing an orchestral track and so far I have written the melody of strings (violins and such) and I want to add trumpets and here are 3 things I consider.

1) Have the trumpets play the same melody as the violins at the exact same octave, is there any reason I should consider this?

2) Have the trumpets play a counter
melody to the violins,

3) Have the trumpets play the same melody of the violins an octave apart.

Does 1 sound good at all?
Which of the above generally will give the best result?

All three would be viable options.

Doubling the parts in unisons or octaves normally would be for bending a unique timbre. Obviously doubling by unison/octave does not create a harmonic change or counterpoint so the main point would be the timbre/tone color.

Can I have..?

If part of your concern is about the counterpoint rule against parallel octaves, that doesn't apply to instrumental doubling as an orchestration technique.

There is nothing theoretically wrong with any of your ideas. Making these types of orchestrating decisions is what an arranger does. Each of these options will create a different effect on the listener. I would recommend creating three different versions, listening to them, and then decided which works best for the situation.

You could even use all three ideas by repeating the melody with a different orchestration each time.

Take a listen to Bolero by Ravel.

You can think of it as a study on varying instrument combinations while maintaining the same the rhythm and melody. You can really start to hear the effect at minute 6:20.

See also: http://theidiomaticorchestra.net/parallel-dobling/

Think of combinations of instruments playing in parallel unisons, fifths, and octaves as though they were kinds of composite instruments. Then an arranger writing for violin, flute, and cello wouldn't have just three instruments at his disposal, but also many more like the univiolinflute, the octaviolinflute, the suboctaviolinflute, the univiolincello, the octaviolincello, the doubloctabiolincello, the octaflutecello, the doubloctaflutecello, etc.

The reason for the rule against parallel octaves or unisons is that having groups of two or more instruments play parallel octaves or unisons will often make it sound as though one has switched to using a different set of instruments. This can be a good thing if done at places where such a switch would make musical sense, but bad if such switches seem to occur willy-nilly without rhyme or reason.

The goal of music theory is not to identify things that are "good" or "bad", but rather to allow composers to identify how things are likely to be perceived. If a composer wants part of a phrase to sound as though it's produced by a univiolinflute while other parts are played by a distinct violin and flute, great--use parallel intervals to achieve that effect. The music theory rules against parallel intervals doesn't say such things are "bad", but rather say that parallel intervals are likely to create such an effect whether the composer wants it or not, and composers should avoid them except when that effect is wanted.

All your proposed schemes are viable, on the surface.
Without seeing what the violinists are playing, having the Trumpet double them is possible given that the parts are not unplayable by a trumpeter. If it's too high in the range, your option of doubling at the octave is possible. If you don't have space for the trumpet to breath you might consider trading lines between members of the trumpet section. If you write in a key that is not particularly trumpet friendly and approachable a professional will certainly tell you. As for melody counter melody.... Go for it, it might work, it might not.

Way outside your context, but in Simon & Garfunkel's "The Boxer", the instrumental break is doubled cornet and pedal steel guitar. There are points where brass aspects stick out, and times where it sounds more steel. I really like it.

Unless you're is a situation where you can't change the orchestration when you get to the practice point, I'd give doubling the melody a shot.