Why does a project like this need three engineers? Two, I can kind of see (a hardware guy and a software guy, though it doesnt look like the hardware guy was doing much here), but three?
One engineer makes a mess because there is nobody to push back. Two engineers either fight or achieve full echo chamber. Three balances some power dynamics, but you definitely don't need 3 FTEs. In fact for this project you probably find 3 of your friends' friends to sit together for 4 hours twice a week and slam out some code or designs, in exchange for a little money and some equity. You even get to write off food and beer as a business expense.
The third one designed the wooden case and the manufacturing process for it ... /s
If the HWE leaves for a bit, same thing. If he's making units production stops, but maybe the SWE can fill in since these dont look like difficult builds.