Thus far I have reproduced this on both PureOS 8.0 and Windows 8.1 with Electrum 3.2.2 and 3 Trezor model ones that are on the most recent firmware 1.6.2
I’m unable to get it into a «partially signed» state — when I go to the Transaction Preview and click Sign, it prompts for my first Trezor, which I plug in and sign. Upon signing, it immediately prompts for the second Trezor, which I plug in and sign. Then it prompts for the third Trezor, I plug in and sign, then it returns to the Preview window.
One other thing work noting is that my key derivation path for the hardware devices is unhardened: m/49/1/0
The Electrum wallet has been successfully finding my received TBTC and spending from them. I’ll test the other scenarios as soon as I can and report back. I’m also happy to provide step by step instructions to reproduce; I already have them written down and would just need to provide you with the seeds.