To answer your first question regarding feminine energies to achieve the same result (I'm assuming by same result you mean a relief from anxiety), I'm not entirely sure. I don't think it's a one-size fits all. It depends on the individual. What works for one may not work for the other. What Shiva showed me was for a specific woman and her situation, but I was also shown how Shiva works in general through that, mainly his organizing and structuring ability of matter, along with his father form. Of course, Shiva also has a destructive form, but that's another topic. Experience can vary depending on the person and what they need at the time. The Gods know what we need and what is best for us at any time when approaching them. They are heavily invested in our advancement! For example, when I work with Lucifer directly, my experience is generally very invigorating, electrifying and energizing, but not always. When he appears in his Infernal guise, it's very primal, awakening, fiery, etc.. When he appears in his Stellar guise, it's more an experience of beauty and awe. Both are enlightening in their own ways, and he often modulates how he manifests depending on what I need at that time, which I allow for and encourage. Some would say that Shiva is a deific or cultural mask of Lucifer, and while I see how that could be true (not saying it isn't, either), I tend to approach them separately.
Back to the feminine... I can only speak from personal experience here. There is a specific Goddess I work with very closely and She never fails to put me at ease! Whatever is bothering me, whatever is troubling me, She is always there to comfort me. She allows me to express whatever it is I am feeling and She makes no judgment whatsoever. She is just present and I can tell Her whatever is bothering me, and Her Love is steadfast, never waning, much like a Mother for Her child. I can also tell Her my successes and triumphs, and everything that is going right, and She is right there with me all the same, still loving me. To me, this is soothing and comforting in a way that feels similar to how the woman I know describes being comforted by Shiva. Perhaps it has to do with the masculine-feminine dynamic and what a persons needs are. This tends to make sense for me. I can't disclose everything, but this woman in particular has also been suffering from loneliness and working on healing issues related to her biological father who abandoned her when she was a teenager. Very traumatic for a young girl. So it's important to keep that in context. Perhaps it is the energy of the Father that she is so lacking, which causes her troubled mind and anxiety, so when he embraces her in his presence, she becomes calm, comforted. Similar for a man who is lonely and anxious from lacking the Mother may feel comforted and soothed in the embrace of the Mother.
For your second question about Shiva as the 3 in 1, your conclusion sounds right to me.