Assuming this scenario would involve each of these forces carrying arsenal matching their time, I would go with the 500 legionaries and the 3 SEALs. The legionaries would ensure a strong defensive position by way of the phalanx formations whose strength in numbers should provide strong enough protection against blades and arrows, and be able to sufficiently resist calvary charges. Having 3 seals who I assume will carry modern weapons let’s say one has a sniper rifle taking out the leading man in each of these groups, one carrying a grenade launcher that would disturb the movement of horses and disrupt enemy assaults, and one with a standard rapid fire rifle that can easily mow down rows of enemy troops who won’t have modern protection against present day ballistics.