posted February 07, 2016 08:47 PM
I am not into Synastry (you would have more chances on that regard in the interpersonal forum) But I'll still try my best.First hand, there's a few points I want to define in order to make the understanding of your grand trine(s) easier
Grand Trine have been marketed almost as a god-blessing or something like that, and I won't claim otherwise, they are a VERY good energy and it's a blessing to have one in a chart, but it's also a double edged knife.
Because of the triangle formation, the energy circulating between the planets is a never-ending one, easily flowing between each focal points so much so that it's excessively easy to become complacent and lazy with the gifts of the Grand Trine. It's an aspect that promotes laziness as much as talent since it lacks the dynamism of the opposition such as what a kite would bring.
Knowing that, a Grand trine where each of the three planets are in water increase the yin-qualities of each signs involved (Cancer - Scorpio - Pisces) which gives you increased intuition, empathy and a depth of feeling that makes it incredibly for you to relate to others. You also have a sort of softness that makes people easily at ease with you. It,s a great gift to have, but not easily understood in the yang-society we have today, what with the focus on performance, success, sometimes at the misery of others.
That's the general idea of a Water Grand Trine and from this point on, I'm tip toeing in areas where I am less comfortable, but I will still try to make it clear for you (bear with me, but still take what I say with a grain of salt as long as there's no one more informed than I confirming my says.)
The Gtrine is happening between Saturn (the responsible and disciplinarian), Pluto (Planet of power and rebirth) and Mercury (The communicator).
I'm of the belief that Pluto and Saturn add great dynamism and bright to your Grand trine, making it less favorable to inertia.
Having Mercury-Saturn-Pluto linked together may give the both of you an innate understanding of how to communicate to each other regarding your respective responsibilities and limits you may have for each other or as partners.
It's great aspect that will help maturity, determination and transformation with each other, giving you almost precognitive intuition as to what you both need to share in order to work with each other.
As it happens in the first, fifth, eight house these innate abilities explained higher in the text are expressed in regard to the matter dealt by these houses. You have the maturity (saturn) to both express yourself to each other (first house) and intuitively know how to communicate your thoughts in regard to matter involving pleasure/children/each others money/sex/etc. (fifth/eight house), sometimes in a spontaneous manner, but always at the right time and in the right way (remember that water grand trine symbolize intuition; you instinctively know when and what to say (mercury) at the right time). This capability that you both share makes you able to go through deep transformation and rebirth regarding each of your views toward the three house in a way that you can both partake in these heavy and deep changes with few problems arising in your partnership since you both instinctively know how to communicate so that the conversation regarding matter of these houses flows easily and is more accepted since you are in tune with each other's feelings.
This is what I can guess from your mutual grand trine, as I said, I'm no expert over this pattern, nor in any kind of synastry. And without the full chart, I cannot fully express the fine tuning of what other things may come and influence this pattern, but that's the general picture I managed to make out of what I had.