Website memory allocation problem

My websites are not loading properly, Its getting error 10 times a day. (Please find the attached screenshots for exact error) When I asked this issue to my hosting providers, they given below mentioned details. "Your sites are using more memory than your server has available. When this happens, the kernel will kill the heaviest user until it can get the memory allocation under control. This will continue until you do one or both of the following: A) Consult an experienced PHP/MySQL developer requesting that they assist you in lowering your site(s) memory footprint B) Purchase an addon/upgrade providing you enough memory to accommodate all of your sites. Until such time as one or both of those are done and your sites are no longer using more memory than your server has access to, you will continue to run into such problems. The likelihood of this causing permanent damage to some of your sites files increases each time it happens." My server has 3 websites which are developed on Wordpress. I am having this issue long time. So please ping me who can solve this problem. Thank you, Manikandan