What is your pay cycle supposed to be? once a month, once a week or every two weeks?
No matter what you are owed money and they should be paying. Honestly you shouldn't have been hired if they didn't have the funds to support your salary/contract. I have sadly been in the shoes of being a founder and having to pay people a few days late and I scrambled and felt horrible that it happened. I also wound up making some tough decisions and cut head count to make sure it wouldn't happen again. Problem being clients don't always pay on time, or pay at all, so you have to be able to support the head count for at least some period of time to cover those times.
The founder may have good intentions, but honestly after 3 years, like others have said, these issues shouldn't happen so something else is up and you need to look for other work likely. Not sure what the rules are in Spain but in the US being late on a payroll if it hasn't yet reached the next pay period won't get you in much trouble (if any), but it is still bad business and a sign to you. And an employee could easily report you to wage and labor. I am not saying I'd cut and run for someone being a little late, but if I had other feelings things weren't right I'd bolt.
Also, the fact you say "we have been promised" makes me thing there are multiple employees in this situation and likely means you are all out of luck and need to find new work.