The three constraints (triple, because they interact) are illustrated below.
More recently, this has been elaborated into a double triangle: attempting to be all things to all people describing project factors, rather than the constraints.
Risk as a factor is interesting...its like putting 'doing your job' as a factor. Risk is assessed then used to guide the setting of budget and schedule against scope. Scope is quality.
I like Glen Alleman's much more helpful diagram, below. He introduces the concept of 'technical performance measure. Much more meaningful then either 'quality' (as if that means anything), or scope.
My preference is for a modified triple constraint, that is more attuned to the business environment and the realities of projects where a project exists to produce a capability that has value for the sponsor.
I've adapted Alleman's TPM to be 'objective', although 'performance' as the out turn result of the project is what we are after. Fussing around with 'quality' as the centre of the triangle does nothing for anyone; what we need to focus on is the value produced (to meet a certain performance by a certain time to produce value from the investment: think Net Present Value).
The money is invested; its not just a cost, but the investment has to be carefully managed to ensure that the sought return achieves the necessary value. We don't manage 'time'. Alleman is right, that we manage schedule, but I prefer to use delivery, as this focuses attention on the project doing what it is meant to do: deliver value.