I want to bake sourdough bread and have created a starter.
It's three days old and bubbling a lot. However it smells like sour yoghurt and not at all yeasty.
Has it gone bad?
Hard to tell for sure, but it sounds like a good sour to me. It has been my experience that sours do not always smell like yeast. Did you add a pinch of yeast in the first place when making the sour? If not, you would be relying on nature to have any natural yeast spores within your sour, and that does not always work out that way.
Either way, if you are only using the sour to impart flavour in your bread, it doesn't really matter if it has yeast in it anyways, as you will add yeast for leavening purposes when you mix the bread.
It has also been my experience that often a 'strange' smelling sour has excellent flavour. Ultimately, it can't hurt to bake a loaf.
When my sourdough is at its best, it floats if I take a table spoon and add to a glass of water. When it is over the hill, it sinks to the bottom. Good news is that an over the hill starter can be great tomorrow night. Just pour most of it out and replenish with flour and water and leave at room temperature.