how can i travel by train from Romania (timisoara ,timispara) to Finland (Helsinki) in just 3days?

At best the whole trip will take about 55 hours and you'll have to change lots of trains, from Romania to Hungary, from Hungary to Ukraine, from Ukraine to Russia and then to Finland.

That is tough. Just going from Romania to Prague, Czech is almost 24 hours but once you get there you are one stop away from West Europe. The train is outdated I am sorry to say. The cheapest way and about the same time is to take the bus system. As long as you get there early enough to get a sit you should be fine. I would suggest taking a bus from Romania to Germany or Denmark and then take a boat. The boats are awsome and a lot of fun in the Baltic.

I'm afraid that is not possible to do in 3 days..

Go see some travel agencies, and do it by plane:)
that would spare you from the fatigue and the stress of chnaging many trains
:)