You don't say how long you've been seeing each other; I'd assume a few months?
There's no need to 'give the impression' you aren't seeing one another-just don't resort to PDA's at work. If he is advocating lying to these 3 to make himself look good and avoid their pettiness, then he has some issues. Lying will bite him in the ass at some point. Does he prefer having these three idolize him and want him?
Best thing-keep quiet at work, make noise at night
I found out that two of my managers at the store I worked at were planning on getting engaged and no one even knew they were seeing each other! It became news only after one got transferred.