Using values from Multiple Sheet

Guys

I have a data in different sheet

example

sheet 1 = A
sheet 2 = B
sheet 3 = C

In all the three sheets i have a data in cell A1 . i want this data to be reflected on the Main Sheet the sheet in which i want the data of all three sheets to be

Like in Main sheet

I have a cell A1 in which value of Sheet A is reflecting
In Cell A2 i want the value of cell A1 of sheet 2 to be reflecting

in order to achive this i have to manually given the references of each sheet to that cell in Main sheet .

Is there any shortcut to achieve this

i have a large data and each sheet name is different like suppose

Apple , black , red , blue

i want to achieve this without doing manual work any suggestions ?

Apart from this

I have another question

When i m working on group sheets , i want to alter the formula which is already present in sheet 1 but when i make alteration in one sheet it dosent alter on the remaining sheet . Unless i erase the entire formula and type the formula again manually

even i tried copy pasting the same formula on the same cell but still not changes are made . please guide