I need root canal under bridge crown. Can I change from replacing three unit bridge to single implant?

In most cases, the root canal procedure can be performed through the bridge crown. So, the bridge does not always need to be removed.If there is decay under the bridge and the bridge needs to be removed, then you have 2 options-Root canal followed by new bridge OR-Root canal and crown for the bridge abutment and crown on the other bridge abutment and single implant for the missing tooth. If you do not want to have the tooth root canal treated, then you can opt to extract the tooth and get implants to replace it and to replace the missing tooth next to it. 

Your question is a little confusing. If you are asking if the bridge can be removed and an implant placed in the space with two crowns on either side, it is very possible. You will still need to have the root canal done in the tooth that is currently requiring one. This will then be crowned with a single crown. The implant , once placed, will need to heal for a minimum of three months before it can be restored with a single crown. Your other option is to either have a new bridge made or use the same bridge if it is still functional.

I find it strange that you had a bridge without performing root canal on the sides. Of course you can replace the bridge implants, but still going to need root canal and crown for the side due to wear made to place the bridge.