I'm not a doctor, so make of this what you will. I can find no evidence that missing a dose of entacapone (especially since you've only been on it for three weeks) will kill you. I frequently miss doses of Stalevo (which contains entacapone) with no ill effects, other than losing the benefit it gives me.
Taking entacapone magnifies the impact of levodopa (one of the components of Madopar) by about 30%. Perhaps your dose now is too large.
What drugs were you on before the change 3 weeks ago, what were the doses and when did you take them? What were your symptoms then?
What drugs are you on now, what are the doses, and what time do you take them?
Remember, you're in charge of your drugs, your drugs aren't in charge of you. The drugs are here to make things better. If they are not doing this, get your doctor to prescribe something different, or instruct you on how to take your present drugs more efficaciously.