looks like one --but not civil war * post civil war (1873) trapdoor type 45 /70 type round is much more likely --the lack of a hollow base , and outside "standing " grooves" rather than" cut into" the body type grooves means post civil war type of "cartridge type" but still blackpowder type round so post civil war cartridge round "bullet" --pre smokeless era most likely. Spanish American war or sooner --1900ish or sooner
The following is intended only as helpful, Educational information.
The answer to your question is no. The generic name "3-ringer" is the relic-hunters' nickname which is applied only to civil war era Minie-ball bullets (or reproductions of them) which have 3 grooves encircling the Minie-ball's body. Your bullet doesn't have 3 grooves (or rings).
Also... Minie-balls are bullets for Muzzleloading rifles. All varieties of Minie-balls have a "significantly large" cavity in their base. Your bullet's base appears to not have a base-cavity, so it isn't a Minie-ball. Your bullet's lack of a base-cavity means it was made for use in a Breechloader firearm.
If you can borrow a Digital Caliper to precisely measure your bullet's diameter, in hundredths-of-an-inch, we might be able to identify it for you. For the moment, all I can be certain about regarding its ID is that it does not resemble any pre-1866 bullet.