question

What is the average temperature and how well is it ventilated?

It is fairly normal for the 3k to derate due to internal temps, in fact I see lower numbers with sustained use down to 1700W in summer.

By adding a forced, temperature based, extractor fan on the inverter outlet we could improve performance a bit.

None of the inverters will sustain max power indefinitely without derating. it is always advised to size above your peak requirements.

In our country temperatures are low now. Inverter is in garage with 14-15 degrees.

I'll try to uncover the case and to put some addition ventilation to check if it will help.

Ok, thank You for suggestion. Uncovering the box solved power drops.

But I don't want inverter to be opened permanently. I prefer to put additional ventilator, and the question is - What to cool inside? Thermal image pointed me to toroidal transformer as the most warm piece of Invertor. As I understood thermal sensor is located inside of it.

Am I right to put some addtitional air to transformer?

...have a look at these...although made here for the MP-II-5000, these 3d printed shrouds are a very interesting concept to enhance cooling (and ambient fan noise), I think -> https://community.victronenergy.com/questions/191916/multiplus-ii-5000-fan-noise-fixed-for-me-without-l.html

Thank you for information. I think about something same. I will check investor's case I will try to put ventilators at the top of case. I there will be enough of holes for wind. I do not want to drill original case even if I have no warranty (I bought this from other person and do not know is warranty is present at all).