@Mar3i 

For outdoor Deco units use Deco X50-Outdoor. This is the only choice.

Can you tell Deco model name for AX3000? Is it X60, X55, X50? This will help with further recommendations.

How many Deco units you will need per floor will depend on floor shape, walls, etc. 350 square meters is about 3800 square feet. For the rectangular place with wooden walls (not brick or cement) I had to put 2 Deco units per 1500 square feet to get excellent WiFi 5 coverage. 

Which means, you may need more than one per floor and could be up to four-five. Obviously, one per floor is minimum, so get initial set of four and then position them on a single floor to figure how many you will actually need per floor. Sorry, but other than practical testing it wouldn't be possible to tell how many will provide level of WiFi coverage you need.

This, i.e. practical testing recommendation, is also applicable to any other brand of WiFi mesh, it is not just for Deco specifically. Do not trust marketing talk telling you one device could cover huge area. Technically it could do that, but under quite special conditions and you might not be happy with how good is actual coverage at the edges of that area.