creating loop for SUM function with variables

[GOR4N]

Screen Shot 2019-12-06 at 04.50.46.png

im trying to create macro that will SUM cells
at the right end cell of columns in G column it should SUM numbers from D:F (green numbers)
at the bottom cells in row 6 it should SUM numbers from 3:5
bottom right cell should SUM green numbers

in this example it's like this

G3.Value = "=SUM(D3:F3)"
G4.Value = "=SUM(D4:F4)"
G5.Value = "=SUM(D5:F5)"

D6.Value = "=SUM(D3:D5)"
E6.Value = "=SUM(E3:E5)"
F6.Value = "=SUM(F3:F5)"

G6.Value = "=SUM(G3:G5)"

this table will grow so macro would be very very helpfull
there is no option to do it manually ... no time for that
i'm trying to avoide human erorr

i have the idea that i should set variables for first&last used row and first&last used column and then to sum only numbers
but my knowleage stops there

one thing that is very important for me is that cell needs to have =SUM function ... i can't use Application.Sum(Range) because sometimes it will happen that i need to change some numbers and i need live SUM update

[davesexcel]

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[GOR4N]

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hey ... tnx ... it's working
BUT it's working only for this scenario
this is going to be dynamic meaning i am going to have more columns and more rows with numbers so is it possible to make variable for columns also?
sometimes it will have 3x3 sometimes 5x10 ... it's not going to be fixed table
rw is looking for last row so how to make macro that will look for last used column and incorporate that in macro?