How damaging can a 3 meter (10 feet) long drain hose be for the washing machine pump and microorganism growth?
Length doesn't matter, going uphill matters
Waste water from anything in your house - sinks, washing machines, dishwashers - doesn't just go 5 or 10 feet. It goes hundreds, typically thousands of feet before it gets to any treatment facility. (OK, if you have a septic tank then the distance is much shorter, but even then, typically a lot more than 10 feet). The main thing is to remember what an HVAC guy once told me (he didn't think highly of &quot;regular&quot; plumbers):

Plumbers just have to know three things:

Hot on the left
Cold on the right
!@$%&amp; runs downhill


If your washing machine had to pump uphill - e.g., if it was in the basement and the only available drain was on the floor above the basement - then the distance would matter as the pump would need enough power to pump up to the drain. In normal installation, the hose only goes up a couple of feet - e.g., to the top of the washing machine - and it is, quite literally, all downhill from there.
I also doubt that, under typical circumstances, a drain hose would be a breeding ground for microorganisms. The exception would be if there is any standing water, which would likely be the case only if there is a loop or flat area of the hose because it is way too long.
If a hose is too long in a practical way - e.g., ends up looped in some fashion, then cut it. If the hose is rubber that should be easy. If it is some other material it still should be possible to cut it and clamp the end in place.