I would not recommend connecting them all to the same connector. Multiple antenna's are often used to help receive a cleaner signal and account for multiple signal bounce paths. If you have 3 internal, you might want to either add 3 external. You could try just running one of them to an external and see if that works but if you are trying to improve reception, you should move all 3. Just remember that if your walls have a lot of metal duct work or some building materials, the wifi may not penetrate well even with an external antenna. Have you thought of adding a 2nd wireless access point in another room? You can use Ethernet over home power adapters if you have no way to run cable. Give the new access point the same ssid and set to a different wifi channel.
(I edited this because autocorrect converted ssid to said & I wanted to correct it.)