If your red is a male, and you just added two more, he may be hiding because platty males can get quite aggressive and while just two might have buddied up, with three, two can become another buddy pair, going after the odd man out.

I've seen that behavor with more than a few platties and the only solution seems to be to keep at least 6, so they will school.

Or, the two buddied up to protect each other from another tank mate that might be viewed by them as a danger. You didn't say what other species are in the tank, but mixed community tanks may not always have the happiest fish in them.

While there are some notable exceptions, most of the commonly kept aquarium fish live in schools in the wild and are not really happy if kept in small numbers in tanks. Generally, if a fish is a schooling species, it's best not to keep any less than 6 per species, so they can gain the support of others and feel safe enough to swim freely about the tank.

Thanks for info birdwidow and I'm running out now to get a few more Platies BUT my Platie started to hide right after his mate died.
I don't know the names of most of my fish, silver dollar, danios and rasboros comes to mind...excuse spelling and some of the little neon ones...they have been all sold to me as "community fish".Never see any fighting nipping etc and have had the tank for 4 years like this.
Could you or somebody else please comment on my Platie hiding right after the death of the other Platie in the tank, before any new fish were put in.

I would provide plants or shale for hiding. Most fish need some sort of refuge.

Fish are very smart animals. Yes, your red is probably depressed about the loss of this buddy of two years. Give him a few weeks, as long as he is eating, and he should be ok. He will make new friends but I do agree that three is an odd number, a school is better to get him back to normal.