awk '
/THEAD/{
p=$0 #store line with 'THEAD'
getline #read next line from file
if(/TCUST/){ #if there is 'TCUST' in it
p=p ORS $0 #add it to 'p' vaiable
getline #and get next line from file
}
if(/IDISC/) #if there is 'IDISC' in line
print p ORS $0 #print 'p' variable altogether with line
}
' test1.dat