Run a cable to each floor, anything less is going to be problematic sooner or later. From this you can add an AP and/or switch.
This +1 million. Flat I moved into has terrible wifi speeds mere metres from the router. Using powerline adapters was a temporary stopgap but even they are only capable of about 40 mbits (theoretically capable of 100, perhaps in a perfect world).
I gave up and just ran CAT5 around the skirting board to a small Netgear prosafe in the living room. Boom, full speeds. I hung an old router set to AP mode purely for wifi range extension and now everything is grand.
Upshot: smart TV hates connecting over wifi, even when it's sat in the same room as the router. Tried numerous routers, Samsung apparently put the worst performing chipset and drivers into Tigen OS. Give it a wired connection: perfect every time, I can max its 100mbit port out on Speedtest (well, the CPU maxes out first).
If you can, trunk some decent shielded CAT6 to every floor. Hang gig switches off those, then you can add whatever wifi repeaters you want and upgrade them in future. All of these mesh networks, wifi repeater kits and so forth are a waste of time and money. Some do double NATting (highly undesirable) unless they're installed as the primary router on the internet connection, and some are overpriced twaddle.
If you're going to get homeplugs, be prepared to spend big bucks - the cheap ones won't manage the speeds you'll expect from a wired connection. Until I dredged out my spare 25 metre CAT5 from the attic, I'd been hovering over the buy button on some TP-Link PA9020s for a while purely because they allegedly are MIMO so hopefully can attain better throughput than the cheaper units.
Costing it though, the time involved in running some cable (if you can tuck it out of the way, or perhaps under carpets etc) is well worth it. I'd rather spend some time and £10 on ethernet cable instead of £100+ on homeplugs with no guarantee of throughput.