i found kneeling up and leaning over the end of the bed was the best position in labour, esp for pushing, i did that for ds2, ds3 and no 5 (dd) i was kind of kneeling when i had ds4 in the pool as well, but not so much.
i am sure you know it all already but yes knees together at all times, be careful getting out of bed and when you bend down, no squatting etc. do you have a support belt from phsyio? or tightish trousers, a good pair of fitted jeans that hold your hips/pelivis tight can be just as good as a support belt.
you might find those bump bands helpful as they help hold the baby weight.
if you labour quickly i would say at first twinge can you call hospital, let them know its starting and to plan on getting pool ready for your arrival, they will see your history of quick labours, but to warn you no 3 can sometimes be different, mine was quick once it got going but lots of false starts, thats common with 3rd babies apparently!
anything that can hlep your core strenght and pelvic floor muscles is worth doing, i found swimming really helped, partly because it takes the weight off of course.
oh and sitting on a birth ball much better than ona sofa as most modern sofas are crap for spd sufferers, not supportive, and you dont sit in a good postion on them generally.