Just want to let you know, peat moss is acidic and by itself can have a pH somewhere between 4 and 5. To remedy this you'll want to mix garden or dolomitic lime into it (a couple heaping tablespoons if it came with no lime whatsoever). If the compost and top soil are rich enough this should do, but a heavy feeding plant might request more nutrients, don't be surprised if it does.
Top soil product characteristics can vary substantially, depending on the source and whether it is bulk or bagged (bagged is generally better in terms of humus and nutrient content). I hope your plan is to use bagged top soil, anyways, as it has generally been tested for these properties, screened and pasteurized or at least checked for pests. Check it out yourself carefully before you use it. It is not generally a good idea to use unprocessed topsoil derived from your backyard or anywhere like that. If at all possible, opt for a topsoil specifically classified as a loam or sandy loam.
Loam is a soil with [roughly] equal proportions of sand, silt and clay; sandy loam is about half sand, a quarter silt and a quarter clay. This makes for an ideal gardening soil that drains well. You don't want to make your soil mix too heavy or compact as this will lead to root problems. To keep it light, avoid adding too much compost and topsoil (esp. if it is a heavier topsoil) as both will contain humus and some amount of clay. Adding perlite or pumice will help lighten the mix and loosen it. Coco coir is another material that can be used along with or instead of sphagnum, it holds both air and water very well and has a more favorable pH.
