I've had personal experience of this recently through my dad and mum. 

My 88 year old dad had a fall outside his own house a few months ago.  He was picked up by a passer by who really wanted to call an ambulance but he refused.  He was OK on that occasion and it was not necessary. 

He and my mum (91 yrs) were both picked up by a neighbour after falling on their drive a few months before that.  Both were fine and no ambulance needed.

But last week he fell in the locked house and could not get up.  The phone fell wth him so he called 999.  He was in a terrible state, barely able to stand before he fell.  He'd been taken to hospital by ambulance but discharged, not admitted, the previous day  He wasn't considered an emergency and the ambulance came after 4 hours.  He's now been in hospital for nearly 3 weeks and stll cannot stand unaided. 

So I'd say you don't always need to call an ambulance after a fall.  You should assess the circumstances. It would have been clear that an ambulance was needed on the third occasion. And if there is doubt then yes call one.  But if the person who fell is certain they are OK and preferably if they won't be alone then no need.