This really sounds a lot like doing J-1 activities as a Visa Waiver Program visitor, which could pose a problem. See, for instance:
UC Berkeley guidelines require all international visitors participating in research activities for 30 days or more to come as J-1 Exchange Visitors. While U.S. immigration inspectors have wide discretion in granting entry to the U.S., they do not view the Visa Waiver Program as appropriate for someone coming to a University to conduct any research especially if it is full-time for an extended period. In addition, many embassies and consulates now require the J-1 visa be used for such purposes. It is recommended that prospective researchers consult with the academic department at UC Berkeley in conjunction with Berkeley International Office to determine the most appropriate status for the proposed visit.
Specifically, it is permissible under the Visa Waiver Program to "conduct independent research (university related research in B status is not allowed)." It sounds like you'd have a hard case arguing that your research is not university-related if you're returning a short time later to do university-related research with your J-1 visa.
It might be allowed for you to make a short visit in January for business and non-research purposes (e.g. meet your collaborators, do some planning, give a talk, etc...), but if you're coming for three months to essentially start your research early, that's more problematic.
Your host institution in the US will have an international scholars & students office that handles immigration matters. I'd consult with them to get their policies and recommendations on how best to handle this.