I have been reading a training book by Brian Clarke. He describes his definitions of these 3 words - and then goes on about training for them. Like most running trainers , his emphasis is mostly upon racing.
He says that "STAMINA" is the ability to finish the distance of a race (forgetting about pace) - "ENDURANCE" is the enduring of the pain of finishing the race at race pace and SPEED is the ability to "kick" at the finish.
His book goes into great length - as they do - but for me his simple further explanation that stamina comes from long slow distance training, endurance comes from tempo training and speed comes from interval training is helpful to me in my understanding of other training plans that I see. He also goes into "strength" ( which relates to pace) by discussing hill workouts.
Speed (as from interval training) is only of use if you are intending to kick across the finishing line (perhaps to beat another competitor) - from my understanding, for my purposes all I need is to do the stamina and endurance training for the type of running that I wish to achieve ( being non-competitive and unlikely to be "racing" somebody across a finish line !!