My DB box is located on the ground floor and I can't get any wifi coverage in my room on the 2nd floor and also on the 3rd floor.

I have heard of Repeaters and Homeplugs as a mean to extend the wifi coverage but each with its own pros and cons but which is the better solution?

Get a long Cat5 cable. Run the cable from the modem to the router place router at strategic point on the 2nd floor and you should be good for the whole building. Make sure that the router supports AC band for high speed wifi. Of course your devices should support it too otherwise it's pointless.

Hmmm.

Preferably not through wired connection because it's quite far from the DB Box to the 2nd Floor's living hall.

I would still go with homeplug...cheap and quick solution. If you have the cash then repeater/AP would be the other alternative

I would go for HomePlug with wireless AP (not wireless extender) since this is the simplest other than LAN cable. However, since your house has 2nd and 3rd floor, I think most likely you are using 3-phase wiring. If that is true, then please note that the HomePlug will not cross to different phase. You will need separate HomePlug for each phase, I believe. Using separate AP on each floor will make your network more scalable/flexible.

but i thought the one condition for Homeplugs to work is that the input and output must be on the same electric supply phase, yes?

remember if your house is 3 story, then homeplug might not work coz not using single phase electric.

Yes, I believe that my suggestion would be the most cost effective for you. While a cable is unsighly, you can easily run it along the wall properly covered/clipped that it'll look neat. One cable would be all you need for the modem to router connection.

Yes....but crossover to other phase seem to work as well, but with reduced speed. My sister's house I got it worked but the connection gets at 10Mb (subscribed to 30M line)...hence its trial and error