dog aggressive lab

i have 3 dogs, a male saint bernard, a female lab mix, and a female chihuahua. the saint and lab get along great but my lab is extremely dog aggressive towards any other dog including my chihuahua. my lab is very protective over me and starting acting like this about 3 years ago when i went into a daycare room with her and several other dogs. thats when it all started. she will be 6 years old on thanksgiving this year. she does great with people but im scared that if my chihuahua would sneak out in the back yard one day my lab would tear her apart. my chi and my lab growl at eachother through the screen door and im not sure how to stop this behavior. i dont want to have to keep them seperated, i would really like to correct this problem. i cant even walk my lab anymore because im terrified a dog off a leash might come up to her, and i know she would attack because she has tried before. any advice on what i can try?