I compared the current one with one from three months ago.
I compared one from now with one from three months ago.
I tried comparing one from now with one from three months ago.
「3ヶ月前と今を比較してみた」は英語で
"I compared one from now with one from three months ago."
又は
"I compared the current one with one from three months ago."となります。
比較する＝compare
「してみた」というニュアンスを強調したいなら
"I tried comparing..." でも大丈夫です。
with one from three months ago ＝ ３ヶ月前のと
ご参考になれば幸いです。
アメリカ合衆国 
