Pull data from separate tabs into one tab

Ok heres my problem im trying to automate my spreadsheet explanation follows.

Say if anything is populated in Sheet1 Column A-C then pull that through to Sheet3 A-C, after i want it to look in Sheet2 if there is anything populated in A-C in Sheet2 then pull that through to the next available row in Sheet3 Column A-C

Any ideas? please help! :D