No, you can’t. Some systems might be able to control full, logical ports—allowing you to substitute one port for another but retaining the same identifier—but individual data pins on an port? Nope. That would be overkill and really not a feature anyone wants since replacing an Ethernet port is a relatively simple fix for those who can—and are willing to—solder a new one on.
Besides, if you reassigned a pin on the connector, then what about the cable itself and whatever connector it terminates to on the other end? That others side would need to be aware of the switch and that’s not something that logistically makes sense.
And regarding this:
If I am right there is need only for 3 pins on Ethernet cable - that would be 100Mbps link.
Nope. An Ethernet cable has 8 pins, but only needs 4 pins for 10/100BT and needs all 8 pins for 1000BT. 3 pins on an RJ45 Ethernet cable = no Ethernet of any kind.