I've just done a back of a fag packet type calculation and reckon you should be entitled to at least a couple of hundred a month, a little more if your eldest was born before April 2017.
Your UC elements/allowances are
Couple £628.10
Child 1 £292.81
Child 2 £292.81
TOTAL £1213.72
If your youngest was born before 6/4/2017, add another £292.81 to that total. Then add either your actual monthly rent or, if you're in a private rental, the LHA rate for a 3-bed in your area, whichever is lower. Add that to the total above to give you your "maximum UC entitlement".
The first £411 of earnings will be disregarded, so £2,089 is the amount of your £2,500 income that will count. The "taper rate" is 55%, so the amount of "tapered earnings" that will be deducted from the "maximum UC entitlement" is £1148.95.
You should get almost all the rent, up to the LHA if that applies.
Disclaimer: I'm very tired and have been crunching numbers all morning, so won't be held responsible for accuracy!
If you're happy to disclose the ages of your children and the LHA rate, I'll run it through the specialist software that I use for work.