Rulings
Q1: Does "if there are 3 or more Digimon" in this card's Main effect count both my Digimon and my opponent's Digimon?
Yes, it does. If there are 3 or more total Digimon between you and your opponent, 1 of your opponent's Digimon gets -12000 DP.
Q2: What does "it further gets -6000 DP" in this card's Main effect mean, exactly?
If the conditions are met, the -6000 DP effect will further get -6000 DP for a total reduction of -12000 DP for 1 of your opponent's Digimon.
Data Sources
EN
Official English Card Listworld.digimoncard.com
Noble Family Arts ST19-15 U LIBERATOR [Main] 1 of your opponent's Digimon gets -6000 DP for the turn. If there are 3 or more Digimon, it further gets -6000 DP. --- [Security] Activate this card's [Main] effects.
A
Official English (Asia) Card Listen.digimoncard.com
Noble Family Arts ST19-15 U LIBERATOR [Main] 1 of your opponent's Digimon gets -6000 DP for the turn. If there are 3 or more Digimon, it further gets -6000 DP. --- [Security] Activate this card's [Main] effects.
W
DCG Wikidigimoncardgame.fandom.com
Noble Family Arts ST19-15 U LIBERATOR [Main] 1 of your opponent's Digimon gets -6000 DP for the turn. If there are 3 or more Digimon, increase the DP reduction of this effect by -6000. --- [Security] Activate this card's [Main] effect.
JP
Official Japanese Card Listdigimoncard.com
ノーブルファミリアーツ ST19-15 U リベレイター 【メイン】ターン終了まで、相手のデジモン1体をDP-6000。デジモンが3体以上いるなら、このDPマイナス効果の数値-6000。 --- 【セキュリティ】このカードの【メイン】効果を発揮する。