I presume the product you are referring to maybe a Penis Pump? If so, they work in the sense that they allow the penis to become more engorged with blood, therefore, making an erection appear bigger. However, this is not a permanant solution and the penis will return to it's original size, once the erection has subsided.
You would do better to spend the money on counselling, to help your husband accept that this is the size of his penis and there is nothing that can realistically done to make it bigger. He needs to build his confidence and self esteem as a lover and not believe his member is holding him back.
You could also help him, by making him feel that his size is not an issue. I noticed in one of your replies, that you referred to his 3 inch erection as "embarrassing". This attitude, jokingly or otherwise can only make him feel worse about himself.