newbie in vhdl

What exactly is the problem? Have you attempted the problems yourself?

Probably some specific examples or error that you observed with test bench would be helpful for understanding your issue.

one of my problems about test bench is the next: Design and Simulate a 12-Bit Magnitude Comparator using a cascade of three 74F85

VHDL CODE (COMPILE GOOD)
library ieee;
use ieee.std_logic_1164.all;
entity comparador_12 is
port( num1 : in std_logic_vector(3 downto 0);  
      num2 : in std_logic_vector(3 downto 0);  
      menor : out std_logic;   
      igual : out std_logic;   
      mayor : out std_logic
       );
end comparador_12;
architecture behavioral of comparador_12 is
begin
    process(num1,num2)
        begin    
            if (num1&num1&num1 > num2&num2&num2 ) then  
                menor <= '0';
                igual <= '0';
                mayor <= '1';
            elsif (num1&num1&num1 < num2&num2&num2) then    
                menor <= '1';
                igual <= '0';
                mayor <= '0';
            else    
                menor <= '0';
                igual <= '1';
                mayor <= '0';            
        end if;
    end process;   
end behavioral;

TEST BENCH
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
entity comparador_12_tb is
end comparador_12_tb;
architecture behavior of comparador_12_tb is
--declaro mis señales de entrada
signal num1,num2 : std_logic_vector(3 downto 0) :=(others => '0');
signal menor,igual,mayor :  std_logic:='0';
begin
--instancia de entidad
UUT : work.comparador_12 port map(num1,num2,menor,igual,mayor);
--definicion del estimulo de proceso
tb : process
begin
num1&num1&num1<="100111000100"; --num1 = 2500
num2&num2&num2<="111011011000"; --num2 = 3800
wait for 10 ns;
num1&num1&num1<="100110011001"; --num1 = 2457
num2&num2&num2<="010111100011"; --num2 = 1507
wait for 10 ns;
num1&num1&num1<="111111111111"; --num1 = 4095
num2&num2&num2<="111111111111"; --num2 = 4095
wait for 10 ns;
num1&num1&num1<="100010001000"; --num1 = 2184
num2&num2&num2<="001101110111"; --num2 = 887
wait;
end process tb;
end;

Please post your code in code tags next time.

You cant do what you are trying to do. you're trying to assign the same signal with 3 different bit patterns.