I suspect you need should recode 1 and 3 to be '1&3' using a factor and then summarise.

But I don't know how to mathematically combine your two proportions because it could mean a lot of different things. You probably need the raw data not the proportion.

If I have a class of children and calculate the proportion with red, brown and blonde hair I can calculate proportion with brown and red hair combined with simple addition.

However, if proportion actually refers to the proportion of people in each category who like carrots then you can not add them together to get a new proportion