PhireSide

modular is ok, specially if you want to go the route you're indicating, but... I do not suggest a 24V solution (28V closer to true peak voltage), if you want to run two 3kW inverters in parallel and you hit 6kW, at 24V (let's call is 25V for ease of maths), 6000 / 25 = 250Ampere? it is doable, but you'd need serious battery size and cabling to go with it, to cater for this kind of current... on the 48V side you'd be down to 120-odd Ampere, which is a lot easier to supply and cable up for...
so, ponder that before making any further investment into the direction you've indicated...
Going modular, maybe you can consider adding solar with a MPPT controller, that can cater for 12, 24 & 48V systems and for now, run it for the 12V side that you already have, then this can also be used if you end up upgrading to 24V or 48V and thus, won't really be money wasted...
HWC to evacuated tubes, is what we did, no heating element as backup and mostly its perfect (or too warm at the moment), on the odd winter morning here in the green Kalahari, with sub-zero temperature (which is not too often) is the hot water, not really hot and less than ideal for a shower... I've been thinking of running the hot water piping through a donkey, so for those rare winter mornings you can heat up the less than hot water though the donkey and have hot water after all, but its been two years and I haven't done this yet, so it probably lets you see that the less than hot water scenario is less off an issue, or rare enough for me not to have done anything about it as yet... (shower early evening and its hardly ever an issue).
Coming back to 3kVA, there are some 48V 3kVA units, but invariably probably on the higher end of the pricing scale, probably not exactly in the 10k or less range...
 

Take a quick look at this 48V for close to your indication of budget. They can also parallel. 
https://shop.thesunpays.co.za/products/sacolar-wifi-compatible-5kva-5kw-pure-sine-wave-axpert-type-inverter-100a-high-voltage-mppt-parallel?variant=39288282087495&currency=ZAR&utm_medium=product_sync&utm_source=google&utm_content=sag_organic&utm_campaign=sag_organic&gclid=Cj0KCQiA54KfBhCKARIsAJzSrdrQoBw_NlhyidpP7YqSIMVvPtMaMH9rm68uvPfOyA_EWjxspLGEZL8aAo_FEALw_wcB

Along those lines, rather spend R2k or R3k or so more on a 5kW 48V inverter, which gives you a much wider choice of batteries. Even some possibility to get a battery at R2K-R3K under your battery budget. Then when you later want to upgrade, instead of a second inverter, you could rather put that money to additional panels or more batteries.