Basic Loop Problem - Nodes in a system

Hi,

Here is my code:

freedof=[Bl,BR,TL,TR]; 
for i=Bl+1:BR-1;
    freedof=[3*i-2 3i-1 3i];
end
for i = BR+1:1:TL-1
end
for i=TL+1:1:TR-1
end

where Bl,BR,TL,TR are previously defined as simple positive numbers. I am trying to create a matrix, for example if Bl = 1 BR = 2 TL = 3 TR = 4 the I need to return a matrix of [1 2 3 4 5 6 7 8 9 10 11 12] i.e 3i-2 3i-1 3i for each defined input (BL etc).

Any help would be much appreciated :) Thanks.