I wanted to purchase life insurance but one of the questions is In the past 3 years have you been treated for cancer. I had cancer back in 2001 and it is in remission so how should I answer this? Since I haven’t had any problems since if I answer no and it comes back and I die from it will they pay the policy or have a problem with it?

The qustion is “have you been TREATED for cancer in the last THREE YEARS” from what you say, the answer is no. If you see a doctor regularly to have your white count checked, then you may want to call an insurance agent for clarification.
Lying on any financial document will usually result in possible nonpayment or forfieture of benefits. If it has been since 2001 and the question is specifically 3 years or more, answer truthfully – which is no, you have not had cancer in the last three years.
Congratulations on being in remission. Being in remission for so long is quite a feat. You must feel really good.
I think you should put it on your application. When the Insurance company asks your doctor for a report it is likely your doctor will mention it. If they disallow death from cancer they will say so when approving your policy. It can be your decision then to go ahead with the policy or not.
I have recently applied for income protection and have a flattened vertebra. I now have income protection but if I lose my income due to a problem with the flattened vertebra – I won’t be protected by the policy. That’s a decision I was happy to make.
It would be a waste of money to pay insurance that your family have to fight tooth and nail to have paid out if you died. It would be easier for your family if they didn’t have to do that.
You should answer no, as it states in the last 3 years. Also try comparing your life insurance to get the best deal. Try theycompare.co.uk, confused.com etc