Putting an ad out for all three, means better tuned applicants, and leaves the company the ability to make an offer of a different position. From the company's position, that's 1.5-3x the number of applicants, for the price of 2 additional ads.

When our company is looking for mixed positions (either a mid-level position where we'll also accept a junior position, or a mixed role such as dba/sysadmin), we'll put out two ads for the position that focus one aspect, then mention the other as a nice-to-have. Then we get candidates who are strong in one area, and average in another, or sometimes (when we're lucky), someone who is strong in both. But either way, we get to choose the best candidate, rather than having good candidates self-select out due to "you must have everything" or super-vague posts.