Was it French composers who wanted to split the Trombones into three different tears? Or was it the Germans?
Q: Just wondering because I thought it was the Germans, however, the name "French Horn" points that it was the French.
This was in the 19th Century btw.
A: I think the asker may be wanting to know who was responsible for dividing the (tenor) trombone section into 3 parts. It was likely the French or perhaps the Italians since the trombone had been used in opera and ballet scores for some time before Beethoven scored for them.
The following quote may give some insight into the question:
"French and Italian composers wrote largely for three tenor trombones, and continued to place the harmony in close position; the Germans remained true to the old group of alto, tenor, and bass, spreading the parts over a rather wider compass." <The History of Orchestration, Adam Carse, p. 249>
Deciding exactly who first used three parts would require the examination of a huge body of early music scores.
Please clarify your question if this is not what your are asking. I'm not sure how the French Horn fits into your question since it is an entirely different instrument from the Trombone.
Musician, composer, teacher.












Horns usually play in a row, except when there are more than, say, six of them (sometimes more than four, depending on the space), they are sometimes in two rows (tiers?) of four. I have never seen three tiers of horns.
Trombones are never tiered. There are only three trombones in a standard symphony orchestra and they sit in a line next to each other. Originally there was an alto trombone, a tenor trombone and a bass trombone in a section (is this possibly the 'tiering' you mention?). It is now usual for there to be two tenors and one bass trombone, although some 18th/19th century parts originally written for the alto trombone are rather high to be played on the tenor trombone and so the alto has made something of a comeback in recent years.