Since I have no idea what you tried, I'm just going to do this in MATLAB.
Note that the first and second file already have trailing empty lines. I'm going to trim those so that there is only one empty line between each chunk of text. This obviously means that this is not a general solution.
A = fileread('1.txt');
B = fileread('2.txt');
C = fileread('3.txt');
fid = fopen('out.txt','w');
fprintf(fid,'%s',[A(1:end-2) B C]);
fclose(fid);
The result is:
nName,nodesC1,nodesC2,nodesC3,nType
n111,0,0,0,structural
n111-y1,0,450,0,structural
n111-y2,0,1500,0,structural
n111-y3,0,2550,0,structural
n121,0,3000,0,structural
n121-x1,600,3000,0,structural
eL,eTy,eSN,eEN,eT
elm.name, grp.name, nod.name(s), mass121, mass1
mass131, mass1, n131, mass141, mass1
mass221, mass2, n221, mass231, mass2
mass241, mass2, n241, mass321, mass1
mass331, mass1, n331, mass341, mass1
col1111, col, n111, n111-y1, nsn1001
resN,resC
nod.name, direction
n111, x+y+z+rx+ry+rz
n111-y1, z+rx+ry
n111-y2, z+rx+ry
n111-y3, z+rx+ry
