If you have played previous Denpa Men games, you will be able to use the Denpa Men you collected and continue to grow your collection in Denpa Men 3. You will be able to start with your previous main party in Denpa Men 3. A little later in the game, you will find a facility called "Summon Laboratory", which will let you bring over the rest of your Denpa Men.

You need to spend coins to summon your previous Denpa Men. When you do, their abilities will slightly increase and your Denpa Men become stronger and join your adventure!