Other answers have mentioned that these are ornaments. As such, try not to think too much about how much they are actually 'worth' in terms of note length. Here is what I hope is some practical advice on playing them. Hopefully it's not too odd!
I agree that it might feel 'right' at first to want to play the three-note ornament before the beat, but as mentioned above, the ornament should 'borrow time' from the note it is attached to (the minim in this case).
You might be able to get a feel for the rhythm by tapping the notes out on the table (or some other surface, the shoulder of your bass for example) with your left hand, with a metronome if you have one. So first try the 'bare' notes, with no ornamentation, to get the general feel of the rhythm. Then, when you're ready, add the three-note ornament by doing a roll with your fingers, like when you drum your fingers on the table but in reverse, starting with your index finger (first note of the ornament) and finishing with your little finger (on the minim).
In the picture below, I've written 1-2-3-4 to correspond to different fingers for each of the notes, obviously this won't be the same as for the fingering of the real notes on the bass - this is purely to get the idea of the rhythm! I hope this makes sense, the x-note heads indicate rhythm rather than any pitch.
Another tip my cello teacher gave me for tricky rhythms was to sniff (breathe in sharply through your nose) when you reach the first note of a bar... So here you might 'sniff' when you start the three-note ornament.