I don't think any goalkeeper would have saved the third goal but the other three were all shots you can imagine Alisson saving and saving with minimal fuss. They certainly weren't goalkeeping errors but could have been saved with slightly better anticipation or positioning (two areas Alisson is absolutely elite at).
I don't think it's a fault of Kelleher but just highlights how much Alisson has bailed us out this season as it's been really common to see teams running right through us.
We'll see if a few new midfielders solves the issue or if it's more systemic than that.
I agree on their third (and the conclusion on Allison), but would probably argue that Southampton's second and fourth should be considered an error on his part (he's not alone to be fair, awful from Firmino Tsimikas and Gomez for the second and Henderson for the fourth).
Second one is right at him, but he dives over it. Fourth one, he was (understandably) high out of his goal - when he retreats he takes up a position that is too wide. If Armstrong took another touch, Kelleher was in the perfect spot - and perhaps that's what he anticipated - but taking it where he did, Kelleher was out of position leaving much of the goal to hit.