I (wrote) these sentences using the helping verb phrase "tend to do":

This makes sense, but it means, "They're expecting me to focus on watching TV, but instead I'm doing something else."

You might be looking for "It's distracting when my family watches TV." (I'm trying to do other things, but my family is watching TV, which is annoying)

#2: It tends to rain heavily and suddenly in Southeast Asian countries in the rainy season.

(Southeast is one word)

This is slightly strange because "heavily" describes the rain itself, but "suddenly" describes how the rainfall starts.

#3: Boy toddlers tend to get sick more than girl toddlers.
Makes sense! Is this really true?

Do they make sense and sound natural?

Yeah! Your use of "tend to" was correct in all cases.