Ten people died and 32 were hurt when a police helicopter crashed into the roof of the Glasgow pub on 29 November 2013.
An investigation found that fuel transfer switches were turned off but it could not answer why the pilot did not land or send a distress call.
The Crown Office said it could not give a timescale for an inquiry.
But a spokesman added that one would be held "as soon as possible".
Families want the Crown Office to begin an inquiry so they can get closure.
Hannah Bennet, from Thompsons Solicitors, which is representing many of the victims and bereaved families, said the delay in beginning an inquiry was causing more pain.
"On the third anniversary of this awful accident a number of our clients are upset and confused as to why no date has yet been announced for the commencement of a fatal accident inquiry (FAI)," she said.
"For many of the victims and their families, closure on the events of that awful night can only truly begin once an FAI has been concluded.
"We are calling on the Crown Office to provide clarity on this issue so that the victims and their families are no longer left wondering when they will get the answers they need and deserve."
Ms Bennet said that while she understood the case was "extremely complex" it was "very unfortunate" there was "still no confirmation" on when the FAI was likely to begin.
The helicopter crew who were killed in the crash were pilot David Traill, PC Tony Collins and PC Kirsty Nelis.
Seven customers in the Clutha died. They were John McGarrigle, Mark O'Prey, Gary Arthur, Colin Gibson, Robert Jenkins, Samuel McGhee and Joe Cusker.
A report published last year by the Air Accidents Investigation Branch said the pilot David Traill did not follow emergency protocol and flew on despite low fuel warnings.
It said fuel transfer pumps were turned off and a controlled landing was not achieved for "unknown reasons".
The report into the crash said:
Alan Crossan, who owns the Clutha bar, told the Good Morning Scotland programme that the failure to provide answers to the tragedy was stopping people from moving forward.
He said: "It's been three years and I didn't expect to be still sitting here looking for the various things to happen like the FAI.
"We've got all sorts of problems that have been tied up in this thing now.
"We've got the FAI and it's still to be decided when we're going to have that. We've got two AAIB reports which are conflicting and we've got families that are still not compensated. That's three years, a long time. It's disgraceful.
"There's been a total lack of information from people and letting us know where things are. If they just let us know 'this is the problems, this is what we're doing, we're investigating the AAIB, we're doing this, we're doing that', but they don't seem to do that."
A Crown Office spokesman said: "The AAIB report into the Clutha tragedy, published last year, raised a number of issues which require further investigation by Police Scotland under the direction of the Crown Office and Procurator Fiscal Service.
"This is a challenging and highly complex enquiry. While we understand that the process may be frustrating for those affected by the tragedy, it is essential that the investigation must be thorough and effective to meet close scrutiny in judicial proceedings.
"Due to the complexity and breadth of the investigation, it is not possible to provide a timescale for any proceedings but a Fatal Accident Inquiry will be held as soon as is possible to allow a full public airing of the evidence."
The Crown Office said it would continue to keep the families advised of progress.
Bereaved families have marked the third anniversary of the Clutha helicopter crash by asking for clarity over when a fatal accident inquiry will be held.