I am unclear on your problem, as Thunderbird has no realistic limit on attachments. What error messages are you receiving?

When I select the three images I want to send, only one lights up. Perhaps it's a problem with my computer? But I never had this problem with Outlook.

Try adding one at a time and see if that works.

Good suggestion, but it still only takes one image. This is a feature that Thunderbird needs to improve.

It's very time inefficient to have to send three photos in three separate emails.

I meant to click to add each photo separately into one email.

Before announcing what needs to be improved, I encourage you to spend a few months using it. Thunderbird has millions of daily users.

I will add that I, and most users, can click as many photos as desired to be added at once. There may be a problem with your setup.