You should ask for what the state sets as the amount. For 3 children, in TX, it is 30% of his net resources. So that will be 30% of his gross income, less taxes (FICA, federal, and state taxes based on a single person), union dues, and expenses for health insurance attributable to the children (i.e. not his own premium--generally it's the "family" rate less the "single" rate if not specified by the plan) and other ordered contributions toward the children's care.
Then you ask for upward deviation for "additional factors," specifically child care expenses incurred in order to work--though unless he agrees to this portion you may be out of luck until you're actually working--and additional needs of the children such as extraordinary educational, health care, or other expenses of the children. You could either itemize the expenses you know you will have every year and ask exactly for his share of those known expenses, or you could leave out extraordinary educational and health care expenses from the monthly child support amount and request that he pay a percentage of the actual amount incurred. The latter does mean that you'd have to work with him on an ongoing basis and depend on him to reimburse you or pay the providers directly in a timely fashion, but it also means that if the costs increase dramatically that you won't have to go back to court to get the amount of his contribution adjusted. You can't just pick some arbitrarily high number, it should be evidenced by historical expenses. The other advantage of having him contribute a percentage toward the actual expense is that he generally won't get a deduction to his net income for the expense like would happen if it were an ordered amount--for example, if he's ordered to contribute $1000 toward daycare costs, his net income would be decreased by $1000 and would mean $300 less child support contribution each month. You could request division of extraordinary costs by income shares, though I suspect you will have to attribute your earnings as at least full-time minimum wage not $0 ($1256:$4000=24%:76%). If your concern about his dependability is whether he'll change his mind in a year, contribution to actual expenses still means it's something he would be ordered to pay. If your concern is that he'll flake on paying on time (but that he'll eventually get around to it) and it will cause you headaches, maybe it's not a viable route or perhaps you could motivate him with language in the agreement that he's responsible to pay with interest or other actual costs incurred as a result of his late payment, if he pays late. In any event, you would include language as to the timeline when he has to pay--a reasonable guideline is 30 days from when he receives the bill (from you).
To order child support for anything except 30% of his net income, the Court must make specific findings for the deviation showing that it's in the best interest of the children. You have to provide the information for why the Court should not follow the statutory guideline amount.
Also, as far as child support is concerned, you're generally not going to be compensated for your time for your children's care, only for actual expenses incurred. That is to say that the time for therapy won't be a justification to increase child support. If ongoing care for the children (in a disability sense, not a choice to SAH) will prevent you from being able to work enough to support your minimum reasonable needs, his contribution to that would be allocated as spousal maintenance, not child support. If disability of the children does not prevent you from working enough to reasonably support yourself and he has not been convicted of committing a violent crime against you/children (I think this is not the case based on your other posts) and you have not been married for 10 years or more, permanent spousal maintenance will not (i.e. may not) be ordered without his consent. Temporary maintenance is much easier to get but, of course, it's limited (generally one month for each year of marriage but could go up to 5 years if you've been married less than 10 years). The maximum amount of monthly maintenance the Court can order is 20% of his monthly gross income, so you can submit evidence for your needs up to that amount along with your justification for the duration. The 11 factors are listed (Sec. 8.052) so focus on these points for your evidence.