It isn’t too late for a phase transfer review meeting, so don’t panic. Some won’t have them until later in the term.
Are they all genuinely your equal preference? If not, you should think carefully before stating all 3 are your preference.
Following the review meeting, the LA will send you the amendment notice within 4 weeks of the meeting. At that point, you can make representations and state your preferred placement.
Are your preferred placements wholly independent? If not, you don’t need an offer of a place from them in order for them to be named in the EHCP. And being full is not defined in law, and for non-wholly independent schools, on its own being ‘full’ is not enough of a reason to refuse to name your preference. The LA has to prove the school is so full admitting DS is incompatible with the efficient education of others or efficient use of resources. The bar for this is higher than many LAs admit. It has to be something tangible and specific and is more than an “adverse effect”, “impact on” or “prejudicial to”.
If the placements are wholly independent, you could speak to them now. Although even some wholly independent schools only accept approaches from the LA. But don’t worry if that applies. Just because a school has replied to some other parents doesn’t mean they will automatically reject any further consultations.
Consultations happen before the EHCP is finalised.
As an aside, following the correct procedure, the advice and information should have been circulated at least 2 weeks before the review meeting. Given the timescales of this thread. That couldn’t have happened. You may wish to go ahead anyway, but if you want more time, you can ask.