Multiple marriages and divorces

The answer by Geremia is correct, IMO. I would like to simply add another way of putting it. Today’s main Gospel reading at mass was about the famous episode of the woman caught in adultery and Jesus save her from a stoning intention (in reality, I just read that stoning for adultery hadn’t been practice for a century at that time; it was a clear provocation against Jesus). The end of the episodes is very clear: when Jesus was ALONE with her and after asking her where her accusers were, Jesus told her that He also did not accuse her BUT she would need to change her life for good “Neither do I condemn thee: go thy way; from henceforth sin no more”.

The message is clear, anybody can understand it. Provided that in such a long time the Church has humanly translated the same message in a correct Church procedure (which includes no valid prior marriage, confession and correct understanding both from you and from the priest who confessed and guided you), you shall approach the sacraments as a new person, look ahead and keep going in the same direction. This is the meaning of the verb “to convert” which comes from the Latin language (con-vertere) and means to literally turn in the opposite direction from where we were coming…

All marriages were not in the Catholic Church.

Assuming you were a baptized Catholic when you attempted marriage outside the Church, those marriage are invalid due to lack of canonical form:

Can. 1108 §1. Only those marriages are valid which are contracted before the local ordinary, pastor, or a priest or deacon delegated by either of them, who assist, and before two witnesses…

^{cf. this Table of Marriage Validity & Liceity}

My 3rd husband

Technically, he was your male concubine, not a husband.

I have already been doing to confession and communion

If you confessed your sins of fornication (if there was sexual activity in those invalid marriages) and disregarding the Church's precept to obey the marriage laws of the Church, then you should be in a state of grace and be able to receive Communion.

How does the Catholic Church explain going to the sacraments in similar situations?

Each person’s return to the practice of the Catholic Faith is unique and does not have to be done alone. One should strive to seek the spiritual light of a good and theological solid minded priest in each individual’s particular case.

There at times, seems to be a myriad of different variables that make this one of those questions almost impossible to have a standard response.

For this reason many priests will have to have recourse to the local canon lawyer of the diocese and/or the diocesan marriage tribunal in order to determine if one or any of your previous marriages were valid.

Although you state that you were married three times, you have not mentioned if your spouses died before you were remarried. These facts must be taken into account.

If you were married between 1983 and 2006, a loophole existed for some mixed marriages to possibly be seen as valid in the Catholic Church.

Marriages of non-Catholic Christians (Protestant marriages)

Baptized Protestants are not bound by the form of marriage, i.e., they do not have to exchange their consent in the presence of a Catholic official. A baptized Catholic who left the church by a formal act and married after the year 1983 (the year the present code of law went into effect) is not bound by the form of marriage either.*

*The motu proprio Omnium in mentem of 26 October 2009 has now been altered, by Pope Benedict XVI and formal acts of defection from the Catholic Church are no longer recognized by the Church. ”After 4 decades” of not being within the fold puts you in this time frame, so you need to truly seek information from diocesan authorities as how to continue. Nevertheless, be courageous, the Church is there to help you on you journey back.

For the rest, one can follow the general counsels given in Geremia’s answer, although I would prefer a more reconciliatory tone, in his response.