How do you pronounce "would've", "should've" and "could've"?

In IPA, that's

would've - /wʊdəv/ - WOULD-uhv
should've - /ʃʊdəv/ - SHOULD-uhv
could've - /kʊdəv/ - COULD-uhv

where "uhv" is a schwa sound followed by a V sound.

Frequently, in fast or informal speech, the V sound will be elided (omitted), and the results will sound like "would-a", "should-a", "could-a".

Edit: I will note that “woulda, shoulda, coulda” (and the same words in different orders) is a very informal idiom that refers dismissively to regretting missed opportunities. The Corpus of Contemporary American English has about 23 examples of all three of them used together, mostly in fiction and spoken English, but also in quoted speech in a few newspaper and magazine articles.

wood-uv, shud-uv, cud-uv.

If you listen closely, sometimes you'll hear me say "wooda". However, I usually say "wood'-uv" with the second syllable very abbreviated.

The pronunciation leads to a common spelling error: would of.