I had a left molar tooth extraction 3 days ago and now my tooth right next to it is very sensitive to touch with a toothbrush. is this normal?

Several Causes: Teeth are extracted for several reasons. If impacted or advanced periodontal bone loss, then some of the root surface on the tooth next to it may be exposed. If extracted for decay, you may have decay on the remaining tooth next to it. See your dentist for diagnosis and treatment.

Yes: The bone around the extraction site is inflamed and may be causing that tooth to be sensitive. Give it some time and things should settle down; you can always have your dentist take a look at it.

Multiple: After an extraction the root surface of the adjacent tooth could be exposed which can cause sensitivity to brushing and cold. Other causes could be the bruising and swelling that follow an extraction or infection. I suggest you use warm salt water rinses several times a day and use sensitive tooth paste.You should follow up with your dentist if the sensitivity persist for more than a week.

No, but common: While sensitivity is not normal, it is a common occurrence. It may be related to the adjacent tooth temporarily having no gum covering the root surface. As the extraction site heals, the sensitivity usually subsides. It may also be related to local inflammation. If it does not get better, have your dentist take a look to make sure no other treatment is necessary.