Striker Emmanuel, 27, scored 19 goals in the National League for Bromley last season and played for the Whites between 2012 and 2014.
Centre-back Howlett-Mundle, 19, has previously had spells at Crystal Palace and Scottish sides Hearts and Montrose.
Forward Jackson Jr, 19, had stints at Whitstable Town and Worthing last season and has agreed a two-year deal.
Meanwhile, 32-year-old former Welling player-manager Loui Fazakerley, who is a fitness coach at Crabble, has been registered as a player for the coming campaign.
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Dover Athletic have re-signed Moses Emmanuel and brought in Jahmal Howlett-Mundle and Ira Jackson Jr.