I am trying to connect ( 1 line ) 2 phone wires ( they are blue and white ) from the wall to telephone socket, but i am not sure where the wires go and how can i identify them and where to connect them. And also can i split the wires and connect them on each cat3 socket?
For a single line tip (red) should be 2, ring (green) should be 3. Blue is not really standard color for phone wiring except when part of 4pr, then blue tip(2), and blue/white ring (3). For verification of your building you would need check termination at the demarc box and any splices, but phone devices are notoriously tolerant of polarity reversal.
Yes you can pigtail and splice phone to feed both jacks.
Your jack plate is not for telephones, it is for internet. It is 8P8C which means "8 pin, 8 conductor" also known as RJ-45. A proper telephone jack is 6P2C or 6P4C, also known as RJ-11. Once you have the proper jack then put the 2 telephone wires to the 2 middle conductors. If the jack shows terrminals 1,2,3,4,5,6 then put the tel line on 3 and 4. It does not matter if blue or white is on pins 3 or 4.