Mix the oil, vinegar and the lemon juice, and rub into each side of the tuna steaks. Lightly season each side with salt and pepper. Marinate in covered dish in refrigerator for 2 hours Minimum, Overnight is best. Place on a hot grill And cook for about 20 seconds each side to seal in the juices. Cook for a further 8 to 10 minutes each side, until the steak is thoroughly hot throughout.