The quickest way to compose is to take classic progression, and use it.
For example i-VII-VII-i in A minor. This progression will loop for 3 minutes.
Then your bass is always the root of the chord. So your bass is always A while on the i, then G while on the VII.
Use the piano or guitar to play that bass note. It should be in the C1-C2 register approximately.
Then the piano and guitar should play the accompaniment and follow the progression, and only play notes belonging to the chord. Nothing else. So they will both play any combination of ACE during i, and GBD during VII. Just place them in any rhythm, order that feels good to you. The range for those should be C3 to C6. Don't go lower than C3, as it will interfere with the bass. And don't go higher than your melody.
As for the flute, it should play the melody, and it should be higher than everything in range. Probably in C4-C6. Whatever sounds best to you. The melody should use mostly the "i" chord notes. That means use mostly ACE. The melody should also use notes that are not ACE, and that are not part of the chord being played. You should alternate between notes that are part of the chord, and notes that are not, and mostly use the notes ACE. So that means over the i chord you should alternate between one of ACE, and between the rest. This will create nice dissonance and resolution. Over the VII chord you should alternate between one of the GBD note and the rest.
As for the rhythm, your rhythm should be the same every bar. If your rhythm is 1/4 + 1/8 + 1/8. Use that same rhythm every single bar. Finds different ways to accentuate this rhythm. But DO NOT move from this main rhythm. Most pieces establish a pulse and never move away from it.
As for your sections. I suggest you use AABAC. That means each A is 4 bars since your progression is 4 bars. So write a A section of 4 bars using the parameters above. Then repeat it again (AA). You can change it slightly or repeat it exactly. Then make a section that contrast very much in the melody (but not in the rest), this is your B section. Then go back to the A section again.
Now you have a 16 bars section (AABA). I suggest you repeat this 16 bars section twice. and then go into the C section. The C section should be very different in the melody and accent mostly the 2 (B) and 7 (G) of the scale, instead of the 1-3-5 (ACE), and different in timbre of the accompaniment, but should play the same chords, and the same rhythm. That C section should last for 4 bars, and be repeated twice. for a total of 8 bars.
So now your song should be 40 bars long. AABA AABA CC. Then I suggest that you repeat those section exactly but by making the timbre more and more dense and intense. More layers, more octaves. Louder. Etc.
At the end your section should look like AABA AABA CC AABA CC AABA
That should last 3 minutes.