convert multiple parts to piano

I would keep one high and one low instrument in voice 1 on their staves and exchange the other two instruments for voice 2. Copy each instrument that are in voice 1 to the appropriate staff. Then one at a time copy the instruments in voice 2 using the selection filter (F6 to display it) and uncheck voice 1 before copying so the rest is not copied. Paste each into the appropriate piano clef and you should be done, except possibly changing the stem directions on some notes if the music lines cross.

If you have a soprano, alto, tenor and bass instruments, the soprano and tenor would stay in voice one. If you have several with the same range you'll have to look at the score to decide which one is best in which voice.

If the parts have similar rhythms, also see Edit / Tools / Implode.