Thanks for your question and asking for me.
The fact some of the income has had CIS deductions made - will mean that you do not owe as much as HMRC estimate you do.
But the first thing you need to do is
1) Contact HMRC and advise them that you are going to sort this out and can then give you a set period of time to do so, before pursuing this further and send you the tax returns you need
2) Gather all income details together from the CIS work you have done (you would have been provided with payment and deduction statements for each month or possibly week from each of the contractors you worked for and also all the details for the umbrella companies you have worked for. which I must advise may cause issue with HMRC as the use of umbrella companies in many instances have been deemed to be non compliant with HMRC legislation, and I imagine you have been under the impression that you have drawn "loans" rather than income, but you do need to gather details of all the monies drawn out.
3) Then gather together all the evidence of expenses you have incurred - so any health and safety wear (such as steel capped boots) or tools that you have purchased etc
4) Then split this into the appropriate tax years so you can complete the tax returns, and you may be best served in engaging a local accountant to undertake this for you, in which case, get onto this as soon as possible, so that when you make contact with HMRC you can advise this to be the case.
HMRC can then assess what you owe, and I am afraid there are likely to be penalties added due to the fact you should have addressed this 3 years ago, and there also may be Class 2 and 4 national insurance to pay.
If what the final figure is, you are unable to afford in one go, then ask HMRC to consider a payment plan for you to spread what you owe over a few months - a year.
But most of all don not worry, just start that ball rolling today, and do feel free to ask any follow up questions.