"A-frame" is good, but how are you using it? I think the suggestions made so far assume that it will be in a static mode in order to get a straight pull up the stairs.
The legs of the A-frame will have to be dug well in. What is the ground like? Is it suitable for the load transmitted by the A-frame legs?
The anchor, a cable to the apex of the A-frame also needs thought. A one ton car on wet grass won't do the job, nor will anything with a dodgy handbrake.
What would you make the A-frame out of? How will you make it?
When moving 3/4 ton uphill, improvisation should only be done by people who know / are trained in what they are doing.
It might be simpler to explain what you are doing at the local plant hire shop and see if they have something designed for the task. The old adage is "use the right tools for the job". Using the right tool for the job might well end up the cheaper option.
Scaffolding planks on the stairs so that things slide up, rather than bump up.
Never, ever stand below whatever you are moving. If you have help, brief your helper(s) to that effect and watch them to make sure they stick to the rule.
Inspect the chains, cables, straps etc before use. Don't use anything that looks even a bit dodgy. What is more important, your safety and £100s / £1000s worth of equipment or a quick trip to the DIY shop for a few £s worth of new rope?