Saying "I love you" means a lot like in any other relationship, specially since I'm always upfront from early days about not saying it lightly like some people do and to remove the cultural ambiguity from our relationship. During such early conversations we found out that for her "I love you" means the same than for us, more than a feeling, for us, it means "I wan't you to stay permanently because I can't be without you and I'd do anything to guard your happiness".
Providing more context as asked:
As we are in a symmetrical type of polyamory (we don't date other people, we commit and stay faithful to whomever is in our relationship), we are flexible on how we date with each other, if one person is not available the other 2 simply go out wherever and the person who was busy is always welcomed to join, we basically share life for the 3 of us. This person is relatively new (almost a year) but has been increasingly staying over at our place, we share everything, we've talked about a future for the 3 of us together, she still has her own apartment though.
My long-standing girlfriend and I had been (not very actively) looking out for other girls since the start, it started very early because she opened up to me about being bisexual, I already knew because we had been friends for years and dated other people before we dated, so I took it as a reminder, a "don't forget I also like girls" type of reminder, to which I was very o.k. with, already had experience anyway. I was clear I didn't like fooling around and she agreed, so anyone else we dated would have to be someone who wanted to be with the both of us. We didn't even had to negotiate, it wasn't even a big deal. We didn't rush into that, we really enjoyed being only the two of us. So, every now and then a person would get close to us but not for long, different expectations, different ideas of what love means and entails, didn't work out. But this person is different, we've all developed a special bond.
I was thinking the right approach would be asking my long-standing girlfriend if she already felt the same, I've already seen all the signs that make evident she's in love with our new companion. We could take her together to a nice place and tell her there, or maybe agree with my girlfriend to tell her separately the same day on different circumstances made special in different ways, and later at night take her to a nice place with the 3 of us to celebrate.
But I really have no experience with that. I'm not sure if that is the best protocol, a different approach could be possible? I don't have the skills, please help.
Please don't answer things like "what if she doesn't say it back" because we don't worry about that. She will say it if she feels the same way and if she still doesn't, we are not putting pressure, there is no need to rush anything, I'm very confident she loves us back though.
Not sure if this helps, but some time ago I was on the other side of the formula, with a slight difference because I'm not bisexual and neither was the man in that relationship, we didn't get that far but we hanged out together and I spent a lot of time at their place. I know from experience being in that position in which you are the one wanting to be in doesn't make you less valuable, I know because when they broke up they sort of fought about whom was going to "keep me". I was very much in love with both of them, I wouldn't have cared if they had told me separately or together as long as the 3 of us stayed together, but that's just me, that's why I'm asking for experienced advice. They ended up telling me separately after they broke up, that was a boomer, heart smashed to smithereens, but that's a whole different story.
How can I tell our new partner "I love you" in a way that does not to ruin her experience of the relationship, or make her feel odd/awkward?