For me, I've learned that nurturing myself is still key to having energy to share with others, so spending time with myself is a good investment in managing multiple partners.
Each relationship also has a "currency" that keeps it viable, and time is only one facet of that. If you know what the currency is, you can leverage it for maximal impact in compact segments of time.
Google calendar is a life saver for multiple-schedule blending.
Being the person in the middle, I rely strongly on my partners to verbally share what they need from me to help them feel valued and loved. This saves a lot of time spent guessing and flailing about ineffectually. (Note: This only works if you are dating real grown-up people
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Try to have a plan B held in reserve, particularly in matters of childcare.