Is it okay to wait for 3 months to get a crown after root canal?

Many factors. A great deal depends on the tooth itself, how it was left, and several other factors. The principal problem is the other teeth around and opposing the root canal tooth. They may shift into the space left by the loss of tooth structure. If it is a front tooth or if the tooth is largely intact, and there is a long term (hard) temporary, you should be OK. If not get a temporary crown.
Not uncommon. However, if you need a crown, you should get a temporary crown or filling ASAP.
Follow DDS advise. Probably not. It is advisable to follow your dentist's advise. You should temporize your tooth ASAP to avoid additional complications, such as fracture, which may lead to extraction and additional expense for an implant.
Discuss. Discuss this with your Dentist. Depends on the integrity of your remaining tooth structure and your occlusion. Delay may allow adjacent and opposing teeth to collapse into area where crown integrity is compromised. Delay may allow deterioration of temporary restoration. Ask your Dentist.