Regarding Possible Cheque Cancellation

My distant relatives have promised me a future payment (after 2-3 years) of a big amount. Since they are not trustable I thought of taking a blank cheque from them having a signature with the issuing date not mentioned. The relatives are very rich (with respect to the payment they have to make me). They agreed to provide me a blank cheque. But my fear is that what if one of the following situations occurs: (i) They cancel the cheque even having sufficient fund in their account to deny paying me stating the reason that I had forced them and took the cheque in an illegal manner ( for example gunpoint) (ii) They declare themselves as bankrupt. (iii) They may adopt any other unsought way to escape paying me and cheat. So what can I do to avoid these risks of me losing my money and live tension free till that period? Thanks!