First, make sure you've consolidated accounts as much as possible. You won't be able to completely of course, but the more that you can the easier it'll be. I'm thinking specifically of the HSA and taxable accounts. Not sure if you could combine HSA, but worth checking.
Next, decide what allocation means functionally. If you implement this allocation in each account individually, then you can probably use broker provided auto re-balancing to take some of the heavy lifting. So if you have 5 accounts total and each is at 50/35/15, then in total your allocation is going to be pretty darn close to that. So you setup to deposit x amount into each account every month, and the broker buys in the 50/35/15 allocation automatically, then quarterly or whatever timing the broker (or you) can go in and rebalance. To mitigate tax consequences in taxable accounts, instead of having it auto buy according to your allocation and thus require more balancing later, you could do it manually and just buy whatever fund is needed to keep your allocation.
Alternatively you could decide to implement the allocation across the whole, and have each account dedicated to one of the funds. So you would only contribute to whichever account(s) based on your desired allocation. The risk here is that you may not fill up your tax advantaged space, depending on market performance.
Also come to terms that your desired allocation is a GOAL. You will rarely, if ever, be exactly on that goal. The actual allocation is going to drift around a bit, and periodically you'll rebalance things to get back on track. You don't want to rebalance too frequently because you'll just drive yourself nuts. Just the thought of it is overwhelming you right now.