Rather than take issue with the very broad generalisations presented by Andyvon as being the way all (or most) HR people operate, I'll attempt to provide an alternative side to this.
Agreed, it is wrong that you are called to a meeting with HR in 3 days time without being told anything about what it is about.
It should not be a disciplinary meeting at this time (unless their process is flawed), because a pre-requisite before a disciplinary meeting is an investigatory meeting, if the process is done according to ACAS recommendations. The normal purpose of such an investigatory meeting is to put certain issues/concerns to you (whatever they may be), then invite you to comment, to hear your side of the story. However it doesn't make sense to say nothing about the purpose of the meeting, or how is anyone expected to do any preparation.
If you are a member of a Union, then do approach your union rep. and he/she
should be willing to attend with you. If not, I do suggest that you bring a friend to enable them to listen to the proceedings and jot notes for you on what is said.
Given that it is only Tuesday am, go back to HR and ask for general information on the purpose of the meeting - pointing out that 'how can you be expected to comment on an issue raised with you on Thursday if given no notice beforehand of the background'. Indeed the very purpose of providing a couple of days' notice is so you can decide whether to bring a colleague or not.