Hi Minifan,
I think if you are booking for March you should be ok for now. I have never seen the sumo in Osaka - I have only been in Tokyo so I have no idea how big the arena is in Osaka nor how many tickets will be available. I usually go in Tokyo and we are fine booking 3 - 4 weeks in advance usually. We do go for the cheapest seats by the way - it's more comfortable to do that trust me! In front of the ring there is no seating area - you get a reserved space on the floor. After 20 minutes or so it gets a little uncomfortable if you're not used to it!
Check this website
Grand Tournament Schedule - Nihon Sumo Kyokai Official Grand Sumo Home Page
I don't think the tickets will be on sale for March yet? They are only just getting ready to sell the January ones - unless Osaka is different.