Solution

    PROGRAM Rom_Num
     IMPLICIT NONE
     CHARACTER(LEN=12) :: Rnum
     INTEGER           :: num, tnum1, tnum
     INTEGER           :: pos
     DO
      Rnum = "            "
      pos = 1
      PRINT*, "Type in your number between 0 and 999 (inclusive)"
      READ*, num
      IF (num .GE. 0 .AND. num .LE. 999) THEN
       tnum1 = num/100
       SELECT CASE (tnum1)
        CASE (1)
         Rnum(pos:pos) = "c"
         pos = pos + 1
        CASE (2)
         Rnum(pos:pos+1) = "cc"
         pos = pos + 2
        CASE (3)
         Rnum(pos:pos+2) = "ccc"
         pos = pos + 3
        CASE (4)
         Rnum(pos:pos+1) = "cd"
         pos = pos + 2
        CASE (5)
         Rnum(pos:pos) = "d"
         pos = pos + 1
        CASE (6)
         Rnum(pos:pos+1) = "dc"
         pos = pos + 2
        CASE (7)
         Rnum(pos:pos+2) = "dcc"
         pos = pos + 3
        CASE (8)
         Rnum(pos:pos+3) = "dccc"
         pos = pos + 4
        CASE (9)
         Rnum(pos:pos+1) = "cm"
         pos = pos + 2
       END SELECT
       tnum1 = tnum1*100
       tnum = (num-tnum1)/10
       SELECT CASE (tnum)
        CASE (1)
         Rnum(pos:pos) = "x"
         pos = pos + 1
        CASE (2)
         Rnum(pos:pos+1) = "xx"
         pos = pos + 2
        CASE (3)
         Rnum(pos:pos+2) = "xxx"
         pos = pos + 3
        CASE (4)
         Rnum(pos:pos+1) = "xl"
         pos = pos + 2
        CASE (5)
         Rnum(pos:pos) = "l"
         pos = pos + 1
        CASE (6)
         Rnum(pos:pos+1) = "lx"
         pos = pos + 2
        CASE (7)
         Rnum(pos:pos+2) = "lxx"
         pos = pos + 3
        CASE (8)
         Rnum(pos:pos+3) = "lxxx"
         pos = pos + 4
        CASE (9)
         Rnum(pos:pos+1) = "xc"
         pos = pos + 2
       END SELECT
       tnum = num-tnum*10-tnum1
       SELECT CASE (tnum)
        CASE (1)
         Rnum(pos:pos) = "i"
         pos = pos + 1
        CASE (2)
         Rnum(pos:pos+1) = "ii"
         pos = pos + 2
        CASE (3)
         Rnum(pos:pos+2) = "iii"
         pos = pos + 3
        CASE (4)
         Rnum(pos:pos+1) = "iv"
         pos = pos + 2
        CASE (5)
         Rnum(pos:pos) = "v"
         pos = pos + 1
        CASE (6)
         Rnum(pos:pos+1) = "vi"
         pos = pos + 2
        CASE (7)
         Rnum(pos:pos+2) = "vii"
         pos = pos + 3
        CASE (8)
         Rnum(pos:pos+3) = "viii"
         pos = pos + 4
        CASE (9)
         Rnum(pos:pos+1) = "ix"
         pos = pos + 2
       END SELECT
       PRINT*, num, "in Roman Numerals is ", TRIM(Rnum)
      ELSE
       PRINT*, "Number out of range"
       EXIT
      END IF
     END DO
    END PROGRAM Rom_Num

1. 888 -- dcclxxxviii
2. 0 --
3. 222 -- ccxxii
4. 536 -- dxxxvi