The lilac bushes in our yard stopped blooming about three years ago. Is there anything we can do to these bushes to revive them?

Have you ever "fed" the plants? I use Osmokote on Jasmine vine and Hibiscus and have had good results. Trimming can also stimulate new fresh flowers. I am assuming the bush is still healthy..

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Tips for Growing Beautiful, Fragrant Lilac

Soil, Sunshine, and Water

Lilacs prefer well-drained, alkaline soil and plenty of sunshine for optimum growth and blooming. If your soil leans toward being acidic, amend it by cultivating some lime into it before planting your lilacs. It's also best to select a site where your lilacs won't be exposed to the wind. Although lilacs love water, they don't enjoy soggy soil. Without proper drainage, lilacs will do little growing and produce fewer blossoms.

Site, Spacing, and Planting

For best results, purchase lilacs from a nursery or garden center where they have been growing in containers. Should you have a large garden area where you want to plant several lilacs of different colors in a group, keep in mind that they will need room to spread. Space larger varieties at least 12-15 feet apart to provide them the spreading room they need. Smaller or dwarf varieties will require less spreading space. Making sure your lilacs have ample room to spread before planting them will prevent future transplanting problems.

Proper Aftercare Promotes Healthy Lilacs

After your lilacs are planted, it is essential to water them regularly during hot summer months. Mulching them with a layer of pine bark will help prevent them from becoming heat-stressed, as mulch helps the soil retain water. Remember not to overwater and always provide adequate drainage.

All plants need nutrients, and lilacs are no exception. Fertilizing with a reliable 5-10-5 fertilizer in the spring is appropriate for lilacs. Check with your local nursery or garden center if you're uncertain about which fertilizer is best for your particular species. Avoid over-fertilizing as this produces more foliage and fewer blossoms.

Removing any spent flower clusters shortly after they have finished blooming is recommended. This prevents them from forming more seeds than flower buds for the following spring. After the blossoms are all spent for the summer, you may wish to prune your lilac bushes lightly just to reshape them. You can also help control your lilac's growth in the first few years by doing very light pruning and shaping. Hard pruning isn't necessary unless you need to reduce the size of your bush or rejuvenate an older plant.

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Lilacs are Beautiful & Fill your Sense of Smell with Delight
Pruning Stimulates Growth…
The main reason to prune flowering shrubs is to maintain a larger portion of the young and vigorous growth wood. Most flower buds are formed on the current or previous year's growth. This makes it important to remove the 1/3 of the oldest wood annually to help keep your flowering shrubs vigorous and producing blooms.

Helps control the size of your flowering shrub
Influences the success of flower buds
Rejuvenates older and overgrown shrubs
Maintains the overall appearance
Maintains the overall health
Encourages growth below the pruning cut

There are several considerations to look at when pruning a flowering shrub including health and overall condition, the season a particular flowering shrub blooms, and what you are trying to accomplish in terms of height or shaping.

Lilacs are springtime Flowering Shrubs:

Pruning spring flowering shrubs is different than summer flowering ones. Spring flowering shrubs produce flower buds off wood produced the previous season. As a result, you will prune these shrubs after they have flowered in the spring and before the next yea's flower buds are set.

By pruning at the wrong time (winter or early spring) you remove many of the flower buds before they ever have a chance to bloom.
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Just some tips I found on the net...

Number one cause for not blooming is lack of sun.

Did the sun conditions change the last three years?

Lilacs need at least 6 hours of sunshine a day. Have your soil tested, if you have acid soil, work in agricultural lime, bone mean or crushed oyster shells to reduce acidity. Water lilacs deeply 1-2 X a week, till late summer when it needs to go dormant. Then, add a layer of compost around bush in fall in In early spring feed lilac a does of fertilizer, But remember if your not blooming, it could be from too much nitrogen, avoid fertilizers high in nitrogen, ask a person at your local nursery which type would be best for your area/soil.
The best way to ensure blooms is to cut the blooms when you do get them, most lilacs bloom from "old wood" meaning you must cut flowers back to a pair of leaves sprouting out if the stem, this is where next years buds will start. If yours isn't blooming and you've fed it and limed the soil, try cutting back the green shoots on the ends of the branches as if you were cutting a bloom.

We have a lilac bush that we would like to get starts from but there are never any sucker shoot around it