According to Jeremy Crawford, "Multiple attacks on the same turn aren't simultaneous, unless a feature or spell says otherwise", the bolts from a high-level Eldritch Blast hit in sequence. You're essentially "hitting" the target 2/3/4 times(assuming you send all three to the same target), hence the separate attack rolls.
The 'spell says otherwise' in this case.
Eldritch Blast has a duration of 'instantaneous'. All the bolts are therefore simultaneous, and any bolt that hits does so in the same instant as every other bolt from that casting of the spell.
If Repelling Blast said, 'When you hit a creature with
eldritch blast, you can push the creature up to 10 feet away from you in a straight line
for each bolt that hits.', then that is what it would do.
But it doesn't say that. It says that if you hit it with
eldritch blast.
Eldritch Blast is a spell. Each bolt within it is
not a separate spell. 'If you hit a creature with *this spell*...'
When you aim, say, two bolts at the same target who is, say, 20 feet away as you cast the spell, each bolt hits at the same time. Each hits the target when it is at 20 feet, each moves the target back 10 feet to a range of 30 feet from you.
The bolts are not aware of each other, and cannot take turns. One bolt cannot politely wait, pausing in mid (instantaneous) air, wait until the first has moved the target to 30 feet, before starting again and hitting the target at 30 feet. No, each bolt does what it says on the tin. It doesn't matter if another bolt is doing the same thing at the same time.
Imagine each bolt teleports the target to Chicago. If two hit, where does the target go? That's right; he goes to Chicago no matter how many bolts hit, because 'sending to Chicago' is what they do.
What Repelling Blast bolts do is:-
a.) hit, or miss
b.) those that hit move you from where you are when they hit you, to 10 feet back from the point where they hit you
c.) if two bolts hit, they hit at the same time. They each hit you when you are at point A. Each has the ability to push you to point B which is 10 feet behind point A
d.) no bolt has the power to push you to point C, which is 20 feet behind point A. When you have been pushed by a bolt or bolts from point A to point B, the instantaneous spell has come and gone. There is no bolt remaining that can hit you at point B, because they both hit you at point A.
You could cast the spell again, of course.
Repelling Blast does not give the spell the ability to push more than 10 feet, no matter the misconception that it pushes 10 feet
for each hit, which it doesn't.