Thank youuu means they strongly want to thank you. They add the extra 3 U for emphasis which is a really sincere thank you. It means they are flattered.
It is usually sent in a text after you someone a favor as a way to say thanks. It is more commonly used by girls than boys.
Another cause is accidentally adding extra u as typos are common especially on a cell phone.
Other variations have 3 or more u’s, for example: Thank Youuuu.
I did some research and found the the more U the less times it was typed. The rarest used 30 U.
The most common was 3 U with 48% occurrence, followed by Thank Youu and Thank Youuuu with 17% and 14%. Here is the data chart: