I have a Renault laguna 1 (1995) 3000 v6 automatic with problem at computer gear box?

this problem was discovered after i went at 4 service renault and only 1 was able to tell me what doesn't work.the gear box when i drive after she change first gear and second one she stacks in third one. another problem before that was the consumption 20l/100km . if someone know about this problems please tell me how i resolve thank you.