You can simulate the 45 degree incline DB press by doing pushups with your feet elevated on a chair or a stool. The only drawback is that no matter how you do the pushups, you can only use bodyweight unless you buy pushup bands. But you can alternate rep ranges, tempo, or hand spacing for great variations.
You can simulate the 45 degree db rows by using a pull-up bar in your doorway that is posted about half-way up. Lie on the floor and place your feet on a chair. Reach up and grab the bar and pull yourself up to the bar, getting a good contraction. Sort of a horizontal pull-up. I’ve done these before and you can really feel a difference. Again, use different hand-spacing, tempos and rep ranges for great effects.
As far as the decline sit-ups, just do lying floor sit-ups with your feet elevated or any other variation.
For the triceps pressdowns, you can go to Wal-Mart.com and search for the Atlus Door Gym??? I think. It costs around 35 dollars and will attach to any doorway. It’s not the best thing in the world, but if you attach all 4 bands to a single handle, you can do some great one-arm pressdowns and laterals among other exercises. Check it out. It may be worth the money if you are not looking to spend much.