Which dog should be best b/w St. Bernard,Neapolitan Mastiff & Boxer,as i'm first time dog owner?

I wanted a dog of lovely,cute & affectionate nature,who should always revolve around me & always be in My Face.I'm a Student of 18 yr & wanted a Male dog of Low shedding,Good Watchdog,Loyal to his family members but also easy to care for & Not too much prone to health issues.I'm having a family with Childrens(Min. of age 7yr),so please tell which breed to opt for (only b/w 3 listed above)as i'm first time Dog Owner.