Here're some ideas on those phrases.
"지금은 소녀시대, 앞으로도 소녀시대, 영원히 소녀시대" => SNSD now, SNSD for the future, SNSD forever.
Taking out the name 소녀시대, one way to say it would be:
"지금도, 앞으로도, 영원히 (소녀시대)" => (SNSD) now, for the future, and forever.
As for 지금은, 앞으로도, 영원히, it has "은" hanging (nothing follows it), which is strange.
도 in 지금도 means "too" or "as well", to make the phrase "both now and for the future, as well as forever".
It makes the meaning catchy and also rhymes with 앞으로도, so it appears to be a good choice.
Hope that gives you an idea.