This could be two things:
if the humidity is low and it is a sunny location the tips will die back as they have the most "edge" area and are at the end of the line for circulation. This can be prevented by putting the plant on a self watering system with a capillary wick and a reservoir of water that it can draw on at all times. Spaths like it moist but not soaking and this can be done with some simple materials
* put a one to two inch layer of inert material like stones in the base of a pot that does not have any holes in the bottom.
* using a knitting needle insert a wicking material such as nylon parachute cord, acrylic or polyester material, into the base of the inner pot leaving six to eight inches to sit in the inert layer.
* fill the outer pot with enough water so the plant is sitting on top of the water but not in it. Capillary action will draw water up to the roots on demand.
- another possible cause of tip browning is a high level of fertilizer in the soil. In the rush to bring product to market many growers fertilize the stock during the entire growing cycle so it looks great on arrival. When you buy the plant and put it in lower light levels with a different watering cycle this can cause tip burn.
This leads me to my last recommendation. If you are watering this plant three times a week this could mean that:
- you are not watering it until water runs out of the bottom of the pot.
- the plant is pot bound and there is not enough soil to retain moisture
- the plant is placed in unusually high light levels
Good practices for spaths are:
- bright filtered light
- when you water add enough water so the entire root ball is soaked
- they respond well to capillary watering which prevents the root ball from drying out and avoids the unfortunate characteristic of these plants to collapse and look dead if they get too dry
I have seen these plants flower in low light, high light, new plants or old. There does not seem to be anything you can do other than good watering practices to encourage them. They will flower when they want to. You can make the flower last longer by removing the spadix, the spiky thing.